refactor: add WebViewGuestDelegate::GetGuestDelegateWeakPtr()

Xref: https://chromium-review.googlesource.com/c/chromium/src/+/4515455

This approach copied from GuestViewBase::GetGuestDelegateWeakPtr() approach in that same commit.

(cherry picked from commit 3f3ab39)

shell/browser/web_view_guest_delegate.cc

@@ -124,4 +124,9 @@ WebViewGuestDelegate::CreateNewGuestWindow(
   return guest_contents;
 }
 
+base::WeakPtr<content::BrowserPluginGuestDelegate>
+WebViewGuestDelegate::GetGuestDelegateWeakPtr() {
+  return weak_ptr_factory_.GetWeakPtr();
+}
+
 }  // namespace electron

shell/browser/web_view_guest_delegate.h

@@ -37,6 +37,8 @@ class WebViewGuestDelegate : public content::BrowserPluginGuestDelegate,
   content::WebContents* GetOwnerWebContents() final;
   std::unique_ptr<content::WebContents> CreateNewGuestWindow(
       const content::WebContents::CreateParams& create_params) final;
+  base::WeakPtr<content::BrowserPluginGuestDelegate> GetGuestDelegateWeakPtr()
+      final;
 
   // WebContentsZoomController::Observer:
   void OnZoomLevelChanged(content::WebContents* web_contents,
@@ -55,6 +57,8 @@ class WebViewGuestDelegate : public content::BrowserPluginGuestDelegate,
   WebContentsZoomController* embedder_zoom_controller_ = nullptr;
 
   api::WebContents* api_web_contents_ = nullptr;
+
+  base::WeakPtrFactory<WebViewGuestDelegate> weak_ptr_factory_{this};
 };
 
 }  // namespace electron