fix: crash with creating OffScreenWebContentsView (#42941)

On the Mac platform, OffScreenWebContentsView uses Automatic Reference
Counting (ARC) to handle the lifecycle of offScreenView_. However, this
private member variable is not initialized and its value is undefined.
In some cases, it is initialized to a garbage value, which may cause ARC
to release the value incorrectly, resulting in a crash.

Co-authored-by: trop[bot] <37223003+trop[bot]@users.noreply.github.com>
Co-authored-by: Jinli Wu <[email protected]>

shell/browser/osr/osr_web_contents_view.h

@@ -110,7 +110,7 @@ class OffScreenWebContentsView : public content::WebContentsView,
   raw_ptr<content::WebContents> web_contents_ = nullptr;
 
 #if BUILDFLAG(IS_MAC)
-  RAW_PTR_EXCLUSION OffScreenView* offScreenView_;
+  RAW_PTR_EXCLUSION OffScreenView* offScreenView_ = nullptr;
 #endif
 };