Pass MouseUp to the captured scrollbar animation controller in layer tree

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 3222 matching lines @@
 }
 
 void LayerTreeHostImpl::MouseDown() {
   ScrollbarAnimationController* animation_controller =
       ScrollbarAnimationControllerForId(scroll_layer_id_mouse_currently_over_);
   if (animation_controller)
     animation_controller->DidMouseDown();
 }
 
 void LayerTreeHostImpl::MouseUp() {
-  ScrollbarAnimationController* animation_controller =
-      ScrollbarAnimationControllerForId(scroll_layer_id_mouse_currently_over_);
-  if (animation_controller)
-    animation_controller->DidMouseUp();
+  for (auto& pair : scrollbar_animation_controllers_)

[bokan, 2017/02/10 15:56:40]

This only needs to notify the one captured scrollbar_animation_controller. Lets
have MouseDown tell the LTHI (via ScrollbarAnimationControllerClient) when a
scrollbar controller is captured. Then MouseUp here can just call DidMouseUp on
the captured controller.

+    pair.second->DidMouseUp();
 }
 
 void LayerTreeHostImpl::MouseMoveAt(const gfx::Point& viewport_point) {
   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
@@ skipping 828 matching lines @@
       worker_context_visibility_ =
           worker_context->CacheController()->ClientBecameVisible();
     } else {
       worker_context->CacheController()->ClientBecameNotVisible(
           std::move(worker_context_visibility_));
     }
   }
 }
 
 }  // namespace cc