fix: fallback to FullSizeContentView for frameless window on mac (#13600)

When using `views::Widget` to create window, Chromium will automatically
use a `BridgedContentView` as content view, which however does not
support draggable regions inside it.

By fallback to `FullSizeContentView` we can work around this problem,
with the price of losing the ability to use `views::View` APIs. Since
we don't expect users to use the new `View` APIs in `BrowserWindow`
anyway, it should not be a problem.

This change does not affect users of `TopLevelWindow`, and for users of
`BrowserWindow` there is nothing to lose.

In the long term we should look into how to make draggable regions work
with `BridgedContentView`. The related Chromium code is still being
changed rapidly, we can wait until Chromium migrated its
`NativeAppWindowCocoa` class to use `views::Widget`.

atom/browser/api/atom_api_browser_window.h

@@ -80,6 +80,10 @@ class BrowserWindow : public TopLevelWindow,
   v8::Local<v8::Value> GetWebContents(v8::Isolate* isolate);
 
  private:
+#if defined(OS_MACOSX)
+  void OverrideNSWindowContentView();
+#endif
+
   // Helpers.
 
   // Called when the window needs to update its draggable region.