Move the IconLoader to use the task scheduler.

This follows https://codereview.chromium.org/2953633002

chromium_src/chrome/browser/icon_loader.cc

@@ -2,8 +2,12 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+#include <utility>
+
 #include "chrome/browser/icon_loader.h"
 #include "base/bind.h"
+#include "base/task_scheduler/post_task.h"
+#include "base/task_scheduler/task_traits.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "content/public/browser/browser_thread.h"
 
@@ -18,10 +22,10 @@ IconLoader* IconLoader::Create(const base::FilePath& file_path,
 
 void IconLoader::Start() {
   target_task_runner_ = base::ThreadTaskRunnerHandle::Get();
-  BrowserThread::PostTaskAndReply(
-      BrowserThread::FILE, FROM_HERE,
-      base::Bind(&IconLoader::ReadGroup, base::Unretained(this)),
-      base::Bind(&IconLoader::OnReadGroup, base::Unretained(this)));
+
+  base::PostTaskWithTraits(
+      FROM_HERE, traits(),
+      base::BindOnce(&IconLoader::ReadGroup, base::Unretained(this)));
 }
 
 IconLoader::IconLoader(const base::FilePath& file_path,
@@ -33,10 +37,6 @@ IconLoader::~IconLoader() {}
 
 void IconLoader::ReadGroup() {
   group_ = GroupForFilepath(file_path_);
-}
-
-void IconLoader::OnReadGroup() {
-  BrowserThread::PostTask(
-      ReadIconThreadID(), FROM_HERE,
-      base::Bind(&IconLoader::ReadIcon, base::Unretained(this)));
+  GetReadIconTaskRunner()->PostTask(
+      FROM_HERE, base::BindOnce(&IconLoader::ReadIcon, base::Unretained(this)));
 }

chromium_src/chrome/browser/icon_loader.h

@@ -12,6 +12,7 @@
 #include "base/files/file_path.h"
 #include "base/macros.h"
 #include "base/single_thread_task_runner.h"
+#include "base/task_scheduler/task_traits.h"
 #include "build/build_config.h"
 #include "content/public/browser/browser_thread.h"
 #include "ui/gfx/image/image.h"
@@ -66,13 +67,19 @@ class IconLoader {
   // Given a file path, get the group for the given file.
   static IconGroup GroupForFilepath(const base::FilePath& file_path);
 
-  // The thread ReadIcon() should be called on.
-  static content::BrowserThread::ID ReadIconThreadID();
+  // The TaskRunner that ReadIcon() must be called on.
+  static scoped_refptr<base::TaskRunner> GetReadIconTaskRunner();
 
   void ReadGroup();
-  void OnReadGroup();
   void ReadIcon();
 
+  // The traits of the tasks posted by this class. These operations may block,
+  // because they are fetching icons from the disk, yet the result will be seen
+  // by the user so they should be prioritized accordingly.
+  static constexpr base::TaskTraits traits() {
+    return {base::MayBlock(), base::TaskPriority::USER_VISIBLE};
+  }
+
   // The task runner object of the thread in which we notify the delegate.
   scoped_refptr<base::SingleThreadTaskRunner> target_task_runner_;
 

chromium_src/chrome/browser/icon_loader_auralinux.cc

@@ -17,10 +17,11 @@ IconLoader::IconGroup IconLoader::GroupForFilepath(
 }
 
 // static
-content::BrowserThread::ID IconLoader::ReadIconThreadID() {
+scoped_refptr<base::TaskRunner> IconLoader::GetReadIconTaskRunner() {
   // ReadIcon() calls into views::LinuxUI and GTK2 code, so it must be on the UI
   // thread.
-  return content::BrowserThread::UI;
+  return content::BrowserThread::GetTaskRunnerForThread(
+    content::BrowserThread::UI);
 }
 
 void IconLoader::ReadIcon() {
@@ -50,6 +51,6 @@ void IconLoader::ReadIcon() {
   }
 
   target_task_runner_->PostTask(
-      FROM_HERE, base::Bind(callback_, base::Passed(&image), group_));
+      FROM_HERE, base::BindOnce(callback_, base::Passed(&image), group_));
   delete this;
 }

chromium_src/chrome/browser/icon_loader_mac.mm

@@ -11,6 +11,7 @@
 #include "base/memory/ptr_util.h"
 #include "base/message_loop/message_loop.h"
 #include "base/strings/sys_string_conversions.h"
+#include "base/task_scheduler/post_task.h"
 #include "base/threading/thread.h"
 #include "ui/gfx/image/image_skia.h"
 #include "ui/gfx/image/image_skia_util_mac.h"
@@ -22,8 +23,9 @@ IconLoader::IconGroup IconLoader::GroupForFilepath(
 }
 
 // static
-content::BrowserThread::ID IconLoader::ReadIconThreadID() {
-  return content::BrowserThread::FILE;
+scoped_refptr<base::TaskRunner> IconLoader::GetReadIconTaskRunner() {
+  // NSWorkspace is thread-safe.
+  return base::CreateTaskRunnerWithTraits(traits());
 }
 
 void IconLoader::ReadIcon() {

chromium_src/chrome/browser/icon_loader_win.cc

@@ -10,6 +10,7 @@
 #include "base/bind.h"
 #include "base/memory/ptr_util.h"
 #include "base/message_loop/message_loop.h"
+#include "base/task_scheduler/post_task.h"
 #include "base/threading/thread.h"
 #include "third_party/skia/include/core/SkBitmap.h"
 #include "ui/display/win/dpi.h"
@@ -30,8 +31,10 @@ IconLoader::IconGroup IconLoader::GroupForFilepath(
 }
 
 // static
-content::BrowserThread::ID IconLoader::ReadIconThreadID() {
-  return content::BrowserThread::FILE;
+scoped_refptr<base::TaskRunner> IconLoader::GetReadIconTaskRunner() {
+  // Technically speaking, only a thread with COM is needed, not one that has
+  // a COM STA. However, this is what is available for now.
+  return base::CreateCOMSTATaskRunnerWithTraits(traits());
 }
 
 void IconLoader::ReadIcon() {