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

cc/trees/layer_tree_host_impl.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_TREES_LAYER_TREE_HOST_IMPL_H_
 #define CC_TREES_LAYER_TREE_HOST_IMPL_H_
 
 #include <stddef.h>
 
 #include <bitset>
@@ skipping 165 matching lines @@
   void ApplyScroll(ScrollNode* scroll_node, ScrollState* scroll_state);
   InputHandlerScrollResult ScrollBy(ScrollState* scroll_state) override;
   void RequestUpdateForSynchronousInputHandler() override;
   void SetSynchronousInputHandlerRootScrollOffset(
       const gfx::ScrollOffset& root_offset) override;
   void ScrollEnd(ScrollState* scroll_state) override;
   InputHandler::ScrollStatus FlingScrollBegin() override;
 
   void MouseDown() override;
   void MouseUp() override;
-  void MouseMoveAt(const gfx::Point& viewport_point) override;
+  void MouseMoveAt(const gfx::Point& viewport_point,
+                   bool is_mouse_down) override;
   void MouseLeave() override;
 
   void PinchGestureBegin() override;
   void PinchGestureUpdate(float magnify_delta,
                           const gfx::Point& anchor) override;
   void PinchGestureEnd() override;
   void StartPageScaleAnimation(const gfx::Vector2d& target_offset,
                                bool anchor_point,
                                float page_scale,
                                base::TimeDelta duration);
@@ skipping 681 matching lines @@
   bool touchpad_and_wheel_scroll_latching_enabled_;
 
   ImplThreadPhase impl_thread_phase_;
 
   DISALLOW_COPY_AND_ASSIGN(LayerTreeHostImpl);
 };
 
 }  // namespace cc
 
 #endif  // CC_TREES_LAYER_TREE_HOST_IMPL_H_