[Media controls] Add rotate-to-fullscreen gesture behind flag

third_party/WebKit/Source/core/dom/ElementVisibilityObserver.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef ElementVisibilityObserver_h
 #define ElementVisibilityObserver_h
 
 #include "core/CoreExport.h"
 #include "core/dom/IntersectionObserver.h"
 #include "platform/heap/Heap.h"
@@ skipping 15 matching lines @@
 class CORE_EXPORT ElementVisibilityObserver final
     : public GarbageCollectedFinalized<ElementVisibilityObserver> {
   WTF_MAKE_NONCOPYABLE(ElementVisibilityObserver);
 
  public:
   using VisibilityCallback = Function<void(bool), WTF::kSameThreadAffinity>;
 
   ElementVisibilityObserver(Element*, std::unique_ptr<VisibilityCallback>);
   virtual ~ElementVisibilityObserver();
 
-  void Start();
+  // The |threshold| is the minimum fraction that needs to be visible.
+  void Start(float threshold = 0.0);
   void Stop();
 
   void DeliverObservationsForTesting();
 
   DECLARE_VIRTUAL_TRACE();
 
  private:
   class ElementVisibilityCallback;
 
   void OnVisibilityChanged(
       const HeapVector<Member<IntersectionObserverEntry>>&);
 
   Member<Element> element_;
   Member<IntersectionObserver> intersection_observer_;
   std::unique_ptr<VisibilityCallback> callback_;
 };
 
 }  // namespace blink
 
 #endif  // ElementVisibilityObserver_h