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

cc/input/scrollbar_animation_controller_thinning.cc

Index: cc/input/scrollbar_animation_controller_thinning.cc
diff --git a/cc/input/scrollbar_animation_controller_thinning.cc b/cc/input/scrollbar_animation_controller_thinning.cc
index 4324a4347bf32943698961ea04265905845f8ff1..4fce163844d8a8567ae7e6622ec87de9225ec15b 100644
--- a/cc/input/scrollbar_animation_controller_thinning.cc
+++ b/cc/input/scrollbar_animation_controller_thinning.cc
@@ -134,6 +134,10 @@ void ScrollbarAnimationControllerThinning::DidMouseMoveNear(float distance) {
   }
 }
 
+bool ScrollbarAnimationControllerThinning::MouseIsNearScrollbar() {
+  return mouse_is_near_scrollbar_;
+}
+
 float ScrollbarAnimationControllerThinning::OpacityAtAnimationProgress(
     float progress) {
   if (opacity_change_ == NONE)