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 74352996fcff1cb964b2685c956ac76819694980..acd7579ed3683bf7253832dc14fb5dba38957318 100644
--- a/cc/input/scrollbar_animation_controller_thinning.cc
+++ b/cc/input/scrollbar_animation_controller_thinning.cc
@@ -66,14 +66,19 @@ void ScrollbarAnimationControllerThinning::RunAnimationFrame(float progress) {
   }
 }
 
-void ScrollbarAnimationControllerThinning::DidCaptureScrollbarBegin() {
+void ScrollbarAnimationControllerThinning::DidMouseDown() {
+  if (!mouse_is_over_scrollbar_)
+    return;
+
   captured_ = true;
   ApplyOpacityAndThumbThicknessScale(1, 1.f);
 }
 
-void ScrollbarAnimationControllerThinning::DidCaptureScrollbarEnd() {
-  captured_ = false;
+void ScrollbarAnimationControllerThinning::DidMouseUp() {
+  if (!captured_)
+    return;
 
+  captured_ = false;
   if (!mouse_is_over_scrollbar_)
     opacity_change_ = DECREASE;
   if (!mouse_is_near_scrollbar_)