fix: crash with creating OffScreenWebContentsView (#42920)

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.

shell/browser/osr/osr_web_contents_view.h

@@ -109,7 +109,7 @@ 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
 };