Implement fade-out animation for Aura overlay scrollbars (CC only).

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..9fad17d4660c44bd7e158a34b902f3cd6bc26c81 100644
--- a/cc/input/scrollbar_animation_controller.cc
+++ b/cc/input/scrollbar_animation_controller.cc
@@ -18,9 +18,9 @@ ScrollbarAnimationController::ScrollbarAnimationController(
     base::TimeDelta resize_delay_before_starting,
     base::TimeDelta duration)
     : client_(client),
+      duration_(duration),
       delay_before_starting_(delay_before_starting),
       resize_delay_before_starting_(resize_delay_before_starting),
-      duration_(duration),
       is_animating_(false),
       scroll_layer_id_(scroll_layer_id),
       currently_scrolling_(false),
@@ -57,10 +57,11 @@ void ScrollbarAnimationController::DidScrollBegin() {
 
 void ScrollbarAnimationController::DidScrollUpdate(bool on_resize) {
   StopAnimation();
-  delayed_scrollbar_fade_.Cancel();
 
   // As an optimization, we avoid spamming fade delay tasks during active fast
   // scrolls.  But if we're not within one, we need to post every scroll update.
+  // TODO(bokan): When do we get ScrollUpdates without a ScrollBegin? This may

[aelias_OOO_until_Jul13, 2016/10/20 23:40:51]

Any kind of main-thread scroll (JS, history or gesture handling).

[bokan, 2016/10/21 00:32:30]

Got it, thanks. Removed TODO and added a test.

+  // be unsupported behavior at this point.
   if (!currently_scrolling_)
     PostDelayedAnimationTask(on_resize);
   else
@@ -94,6 +95,7 @@ void ScrollbarAnimationController::StartAnimation() {
 }
 
 void ScrollbarAnimationController::StopAnimation() {
+  delayed_scrollbar_fade_.Cancel();
   is_animating_ = false;
 }