Media Controls: move timeline related code to modules.

third_party/WebKit/Source/modules/media_controls/elements/MediaControlElementsHelper.h

 // Copyright 2017 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 MediaControlElementsHelper_h
 #define MediaControlElementsHelper_h
 
 #include "platform/wtf/Allocator.h"
 
 namespace blink {
 
 class Event;
+class LayoutObject;
 
 // Helper class for media control elements. It contains methods, constants or
 // concepts shared by more than one element.
 class MediaControlElementsHelper final {
   STATIC_ONLY(MediaControlElementsHelper);
 
  public:
   static bool IsUserInteractionEvent(Event*);
+
+  // Sliders (the volume control and timeline) need to capture some additional
+  // events used when dragging the thumb.
+  static bool IsUserInteractionEventForSlider(Event*, LayoutObject*);
 };
 
 }  // namespace blink
 
 #endif  // MediaControlElementsHelper_h