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

cc/trees/layer_tree_host_impl.cc

 // 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.
 
 #include "cc/trees/layer_tree_host_impl.h"
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <algorithm>
@@ skipping 3469 matching lines @@
         ScrollbarAnimationControllerForElementId(
             scroll_element_id_mouse_currently_captured_);
 
     scroll_element_id_mouse_currently_captured_ = ElementId();
 
     if (animation_controller)
       animation_controller->DidMouseUp();
   }
 }
 
-void LayerTreeHostImpl::MouseMoveAt(const gfx::Point& viewport_point) {
+void LayerTreeHostImpl::MouseMoveAt(const gfx::Point& viewport_point,
+                                    bool is_mouse_down) {
   gfx::PointF device_viewport_point = gfx::ScalePoint(
       gfx::PointF(viewport_point), active_tree_->device_scale_factor());
   LayerImpl* layer_impl =
       active_tree_->FindLayerThatIsHitByPoint(device_viewport_point);
 
   // Check if mouse is over a scrollbar or not.
   // TODO(sahel): get rid of this extera checking when
   // FindScrollNodeForDeviceViewportPoint finds the proper node for scrolling on
   // the main thread when the mouse is over a scrollbar as well.
   ElementId scroll_element_id;
@@ skipping 22 matching lines @@
       old_animation_controller->DidMouseLeave();
     }
     scroll_element_id_mouse_currently_over_ = scroll_element_id;
   }
 
   ScrollbarAnimationController* new_animation_controller =
       ScrollbarAnimationControllerForElementId(scroll_element_id);
   if (!new_animation_controller)
     return;
 
-  new_animation_controller->DidMouseMove(device_viewport_point);
+  new_animation_controller->DidMouseMove(device_viewport_point, is_mouse_down);
 }
 
 void LayerTreeHostImpl::MouseLeave() {
   for (auto& pair : scrollbar_animation_controllers_)
     pair.second->DidMouseLeave();
   scroll_element_id_mouse_currently_over_ = ElementId();
 }
 
 void LayerTreeHostImpl::PinchGestureBegin() {
   pinch_gesture_active_ = true;
@@ skipping 755 matching lines @@
 
 void LayerTreeHostImpl::ShowScrollbarsForImplScroll(ElementId element_id) {
   if (!element_id)
     return;
   if (ScrollbarAnimationController* animation_controller =
           ScrollbarAnimationControllerForElementId(element_id))
     animation_controller->DidScrollUpdate();
 }
 
 }  // namespace cc