fix: no more need to distroy WebContents asyncronously

Because app.quit now ensures pending tasks are finished before exiting.

shell/browser/api/atom_api_web_contents.cc

@@ -560,12 +560,9 @@ WebContents::~WebContents() {
       // BrowserWindow destroys WebContents asynchronously, manually emit the
       // destroyed event here.
       WebContentsDestroyed();
-    } else if (Browser::Get()->is_shutting_down()) {
-      // Destroy WebContents directly when app is shutting down.
-      DestroyWebContents(false /* async */);
     } else {
-      // Destroy WebContents asynchronously unless app is shutting down,
-      // because destroy() might be called inside WebContents's event handler.
+      // Destroy WebContents asynchronously since destroy() might be called
+      // inside WebContents's event handler.
       DestroyWebContents(!IsGuest() /* async */);
       // The WebContentsDestroyed will not be called automatically because we
       // destroy the webContents in the next tick. So we have to manually