Implement autoplay gesture override experiment.

Source/core/testing/Internals.cpp

 /*
  * Copyright (C) 2012 Google Inc. All rights reserved.
  * Copyright (C) 2013 Apple 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:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
@@ skipping 2462 matching lines @@
     ScrollableArea* scrollableArea = layer->scrollableArea();
     if (!scrollableArea)
         return false;
     ScrollAnimator* animator = layer->scrollableArea()->scrollAnimator();
     if (!animator)
         return false;
 
     return animator->setScrollbarsVisibleForTesting(visible);
 }
 
+void Internals::triggerAutoplayVisibilityCheck(HTMLMediaElement* element)
+{
+    element->visibilityTimerFired(0);
+}
+
 } // namespace blink