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

cc/input/scrollbar_animation_controller.cc

Index: cc/input/scrollbar_animation_controller.cc
diff --git a/cc/input/scrollbar_animation_controller.cc b/cc/input/scrollbar_animation_controller.cc
index 45feb3ae2bfb0fa0e664b49d8531652de8dc0437..2ecea4708d66fa47cb1946c307d5e81a9c18b4af 100644
--- a/cc/input/scrollbar_animation_controller.cc
+++ b/cc/input/scrollbar_animation_controller.cc
@@ -44,6 +44,10 @@ bool ScrollbarAnimationController::Animate(base::TimeTicks now) {
   return true;
 }
 
+bool ScrollbarAnimationController::MouseIsNearScrollbar() {
+  return false;
+}
+
 float ScrollbarAnimationController::AnimationProgressAtTime(
     base::TimeTicks now) {
   base::TimeDelta delta = now - last_awaken_time_;