If a compositing update is declared as needed, scheduleAnimation should be triggered.

public/web/WebWidgetClient.h

 /*
  * Copyright (C) 2009 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 83 matching lines @@
     // frame have been issued.
     virtual void didCommitAndDrawCompositorFrame() { }
 
     // Called for compositing mode when swapbuffers has been posted in the GPU
     // process.
     virtual void didCompleteSwapBuffers() { }
 
     // Called when a call to WebWidget::animate is required
     virtual void scheduleAnimation() { }
 
+    // Called to query the state of the rendering back-end. Should return true
+    // when scheduleAnimation (or possibly some other cause for another frame)
+    // was called, but before WebWidget::animate actually does a frame.
+    virtual bool isCompositorFramePending() const { return false; }
+
     // Called when the widget acquires or loses focus, respectively.
     virtual void didFocus() { }
     virtual void didBlur() { }
 
     // Called when the cursor for the widget changes.
     virtual void didChangeCursor(const WebCursorInfo&) { }
 
     // Called when the widget should be closed.  WebWidget::close() should
     // be called asynchronously as a result of this notification.
     virtual void closeWidgetSoon() { }
@@ skipping 63 matching lines @@
     // of the touch actions that are permitted for this touch.
     virtual void setTouchAction(WebTouchAction touchAction) { }
 
 protected:
     ~WebWidgetClient() { }
 };
 
 } // namespace blink
 
 #endif