Don't fade in overlay scrollbar when user interacting the content under scrollbar.

cc/input/input_handler.h

 // Copyright 2011 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 CC_INPUT_INPUT_HANDLER_H_
 #define CC_INPUT_INPUT_HANDLER_H_
 
 #include <memory>
 
 #include "base/macros.h"
@@ skipping 140 matching lines @@
   // root layer, and the layer can no longer move, the root overscroll
   // accumulated within this ScrollBegin() scope is reported in the return
   // value's |accumulated_overscroll| field. Should only be called if
   // ScrollBegin() returned SCROLL_STARTED.
   virtual InputHandlerScrollResult ScrollBy(ScrollState* scroll_state) = 0;
 
   // Returns SCROLL_STARTED if a layer was actively being scrolled,
   // SCROLL_IGNORED if not.
   virtual ScrollStatus FlingScrollBegin() = 0;
 
-  virtual void MouseMoveAt(const gfx::Point& mouse_position) = 0;
+  virtual void MouseMoveAt(const gfx::Point& mouse_position,
+                           bool mouse_down) = 0;

[bokan, 2017/06/08 14:02:38]

Can we just keep track of the mouse state from the ScrollbarAnimationController?
That is, we already get MouseDown and MouseUp, just keep track of the button
state using those. I realize it's slightly different behavior since if you start
a click in one scroller and drag outside of it other scrollers can fade-in their
scrollbars. But I think that's ok and not worth the added complexity. Those
faded in scrollbars wont prevent you from dragging/selecting under them. 

   virtual void MouseDown() = 0;
   virtual void MouseUp() = 0;
   virtual void MouseLeave() = 0;
 
   // Stop scrolling the selected layer. Should only be called if ScrollBegin()
   // returned SCROLL_STARTED.
   virtual void ScrollEnd(ScrollState* scroll_state) = 0;
 
   // Requests a callback to UpdateRootLayerStateForSynchronousInputHandler()
   // giving the current root scroll and page scale information.
@@ skipping 52 matching lines @@
   InputHandler() {}
   virtual ~InputHandler() {}
 
  private:
   DISALLOW_COPY_AND_ASSIGN(InputHandler);
 };
 
 }  // namespace cc
 
 #endif  // CC_INPUT_INPUT_HANDLER_H_