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/chrome/browser_delegate.h

@@ -126,6 +126,9 @@ class BrowserDelegate : public content::WebContentsDelegate {
   // Set the draggable region relative to web contents.
   // Called from DraggableRegionsHostImpl::UpdateDraggableRegions.
   virtual void UpdateDraggableRegion(const SkRegion& region) {}
+
+  // Called at the end of a fullscreen transition.
+  virtual void WindowFullscreenStateChanged() {}
 };
 
 }  // namespace cef