Iframe/div MD scrollbars get idle when mouse leaves the nested elements.

cc/input/scrollbar_animation_controller_thinning.cc

 // Copyright 2013 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/input/scrollbar_animation_controller_thinning.h"
 
 #include "base/memory/ptr_util.h"
 #include "base/time/time.h"
 #include "cc/layers/layer_impl.h"
 #include "cc/layers/scrollbar_layer_impl_base.h"
@@ skipping 116 matching lines @@
 
     if (mouse_is_near_scrollbar_ != mouse_is_near_scrollbar) {
       mouse_is_near_scrollbar_ = mouse_is_near_scrollbar;
       thickness_change_ = mouse_is_near_scrollbar_ ? INCREASE : DECREASE;
     }
 
     StartAnimation();
   }
 }
 
+bool ScrollbarAnimationControllerThinning::MouseIsNearScrollbar() {
+  return mouse_is_near_scrollbar_;
+}
+
 float ScrollbarAnimationControllerThinning::OpacityAtAnimationProgress(
     float progress) {
   if (opacity_change_ == NONE)
     return mouse_is_over_scrollbar_ ? 1.f : kIdleOpacity;
   float factor = opacity_change_ == INCREASE ? progress : (1.f - progress);
   float ret = ((1.f - kIdleOpacity) * factor) + kIdleOpacity;
   return ret;
 }
 
 float ScrollbarAnimationControllerThinning::
@@ skipping 49 matching lines @@
           property_trees->effect_id_to_index_map[scrollbar->id()],
           scrollbar->layer_tree_impl());
     }
     scrollbar->SetThumbThicknessScaleFactor(AdjustScale(
         thumb_thickness_scale, scrollbar->thumb_thickness_scale_factor(),
         thickness_change_, kIdleThicknessScale, 1));
   }
 }
 
 }  // namespace cc