Implement autoplay gesture override experiment.

Source/core/testing/Internals.h

 /*
  * 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 354 matching lines @@
 
     String unscopeableAttribute();
     String unscopeableMethod();
 
     ClientRectList* focusRingRects(Element*);
 
     void setCapsLockState(bool enabled);
 
     bool setScrollbarVisibilityInScrollableArea(Node*, bool visible);
 
+    void triggerAutoplayVisibilityCheck(HTMLMediaElement*);
+
 private:
     explicit Internals(ScriptState*);
     Document* contextDocument() const;
     LocalFrame* frame() const;
     Vector<String> iconURLs(Document*, int iconTypesMask) const;
     ClientRectList* annotatedRegions(Document*, bool draggable, ExceptionState&);
 
     DocumentMarker* markerAt(Node*, const String& markerType, unsigned index, ExceptionState&);
     Member<InternalRuntimeFlags> m_runtimeFlags;
 
     IterationSource* startIteration(ScriptState*, ExceptionState&) override;
 };
 
 } // namespace blink
 
 #endif // Internals_h