[Media Controls] Add UMA for timeline scrubber

third_party/WebKit/Source/core/html/shadow/SliderThumbElement.h

 /*
  * Copyright (C) 2006, 2007, 2008, 2009 Apple Inc. All rights reserved.
  * Copyright (C) 2010 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.
@@ skipping 50 matching lines @@
  private:
   SliderThumbElement(Document&);
   LayoutObject* createLayoutObject(const ComputedStyle&) override;
   Element* cloneElementWithoutAttributesAndChildren() override;
   bool isDisabledFormControl() const override;
   bool matchesReadOnlyPseudoClass() const override;
   bool matchesReadWritePseudoClass() const override;
   Node* focusDelegate() override;
   void startDragging();
 
-  bool m_inDragMode;
+  bool m_inDragMode;  // Mouse only. Touch is handled by SliderContainerElement.
 };
 
 inline Element* SliderThumbElement::cloneElementWithoutAttributesAndChildren() {
   return create(document());
 }
 
 // FIXME: There are no ways to check if a node is a SliderThumbElement.
 DEFINE_ELEMENT_TYPE_CASTS(SliderThumbElement, isHTMLElement());
 
 class SliderContainerElement final : public HTMLDivElement {
@@ skipping 21 matching lines @@
 
   bool m_hasTouchEventHandler;
   bool m_touchStarted;
   Direction m_slidingDirection;
   LayoutPoint m_startPoint;
 };
 
 }  // namespace blink
 
 #endif