chrome: Standardize fullscreen transition behavor/callbacks (fixes #3571)

Use the same code path for all fullscreen transitions so that Chrome UI updates
correctly. All user-initiated fullscreen transitions now result in
CefWindowDelegate::OnWindowFullscreenTransition callbacks.

libcef/browser/views/native_widget_mac.h

@@ -48,6 +48,9 @@ class CefNativeWidgetMac : public views::NativeWidgetMac {
   const CefRefPtr<CefWindow> window_;
   CefWindowDelegate* const window_delegate_;
 
+  // Returns true if the CefWindow is fully initialized.
+  bool IsCefWindowInitialized() const;
+
   BrowserView* browser_view_ = nullptr;
 };