migrate off QuitCurrent*Deprecated in browser.cc

modelled after https://chromium-review.googlesource.com/c/chromium/src/+/1096475 and https://chromium-review.googlesource.com/c/chromium/src/+/1096475

atom/browser/atom_browser_main_parts.cc

@@ -260,6 +260,11 @@ bool AtomBrowserMainParts::MainMessageLoopRun(int* result_code) {
   return brightray::BrowserMainParts::MainMessageLoopRun(result_code);
 }
 
+void AtomBrowserMainParts::PreDefaultMainMessageLoopRun(
+    base::OnceClosure quit_closure) {
+  Browser::SetMainMessageLoopQuitClosure(std::move(quit_closure));
+}
+
 void AtomBrowserMainParts::PostMainMessageLoopStart() {
   brightray::BrowserMainParts::PostMainMessageLoopStart();
 #if defined(OS_POSIX)

atom/browser/atom_browser_main_parts.h

@@ -81,6 +81,7 @@ class AtomBrowserMainParts : public brightray::BrowserMainParts {
   void ToolkitInitialized() override;
   void PreMainMessageLoopRun() override;
   bool MainMessageLoopRun(int* result_code) override;
+  void PreDefaultMainMessageLoopRun(base::OnceClosure quit_closure) override;
   void PostMainMessageLoopStart() override;
   void PostMainMessageLoopRun() override;
 #if defined(OS_MACOSX)

atom/browser/browser.cc

@@ -6,6 +6,7 @@
 
 #include <memory>
 #include <string>
+#include <utility>
 
 #include "atom/browser/atom_browser_main_parts.h"
 #include "atom/browser/browser_observer.h"
@@ -14,6 +15,7 @@
 #include "atom/browser/window_list.h"
 #include "base/files/file_util.h"
 #include "base/message_loop/message_loop.h"
+#include "base/no_destructor.h"
 #include "base/path_service.h"
 #include "base/run_loop.h"
 #include "base/threading/thread_restrictions.h"
@@ -23,6 +25,9 @@
 
 namespace atom {
 
+// Null until/unless the default main message loop is running.
+base::NoDestructor<base::OnceClosure> g_quit_main_message_loop;
+
 Browser::LoginItemSettings::LoginItemSettings() = default;
 Browser::LoginItemSettings::~LoginItemSettings() = default;
 Browser::LoginItemSettings::LoginItemSettings(const LoginItemSettings& other) =
@@ -90,9 +95,8 @@ void Browser::Shutdown() {
   for (BrowserObserver& observer : observers_)
     observer.OnQuit();
 
-  if (base::ThreadTaskRunnerHandle::IsSet()) {
-    base::ThreadTaskRunnerHandle::Get()->PostTask(
-        FROM_HERE, base::RunLoop::QuitCurrentWhenIdleClosureDeprecated());
+  if (*g_quit_main_message_loop) {
+    std::move(*g_quit_main_message_loop).Run();
   } else {
     // There is no message loop available so we are in early stage.
     exit(0);
@@ -191,6 +195,10 @@ void Browser::PreMainMessageLoopRun() {
   }
 }
 
+void Browser::SetMainMessageLoopQuitClosure(base::OnceClosure quit_closure) {
+  *g_quit_main_message_loop = std::move(quit_closure);
+}
+
 void Browser::NotifyAndShutdown() {
   if (is_shutdown_)
     return;

atom/browser/browser.h

@@ -239,6 +239,10 @@ class Browser : public WindowListObserver {
 
   void PreMainMessageLoopRun();
 
+  // Stores the supplied |quit_closure|, to be run when the last Browser
+  // instance is destroyed.
+  static void SetMainMessageLoopQuitClosure(base::OnceClosure quit_closure);
+
   void AddObserver(BrowserObserver* obs) { observers_.AddObserver(obs); }
 
   void RemoveObserver(BrowserObserver* obs) { observers_.RemoveObserver(obs); }