Overlay scrollbars shouldn't fade out while mouse is over/near.

BUG=669668
TEST=ScrollbarAnimationControllerThinningTest.DontFadeAfterReleasedNear,
ScrollbarAnimationControllerThinningTest.FadeAfterReleasedFar,
ScrollbarAnimationControllerThinningTest.MoveNearAndDontFadeOut,
ScrollbarAnimationControllerThinningTest.MoveOverAndDontFadeOut
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_trusty_blink_rel

Review-Url: https://codereview.chromium.org/2552813002
Cr-Commit-Position: refs/heads/master@{#437585}

cc/input/scrollbar_animation_controller_thinning.cc

@@ -72,7 +72,8 @@ void ScrollbarAnimationControllerThinning::RunAnimationFrame(float progress) {
     if (current_animating_property_ == THICKNESS) {
       thickness_change_ = NONE;
       SetCurrentAnimatingProperty(OPACITY);
-      PostDelayedAnimationTask(false);
+      if (!mouse_is_near_scrollbar_)
+        PostDelayedAnimationTask(false);
     }
   }
 }
@@ -107,7 +108,6 @@ void ScrollbarAnimationControllerThinning::DidMouseUp() {
     StartAnimation();
   } else {
     SetCurrentAnimatingProperty(OPACITY);
-    PostDelayedAnimationTask(false);
   }
 }
 
@@ -135,6 +135,18 @@ void ScrollbarAnimationControllerThinning::DidScrollUpdate(bool on_resize) {
   ApplyThumbThicknessScale(mouse_is_near_scrollbar_ ? 1.f
                                                     : kIdleThicknessScale);
   SetCurrentAnimatingProperty(OPACITY);
+
+  // Don't fade out the scrollbar when mouse is near.
+  if (mouse_is_near_scrollbar_)
+    StopAnimation();
+}
+
+void ScrollbarAnimationControllerThinning::DidScrollEnd() {
+  ScrollbarAnimationController::DidScrollEnd();
+
+  // Don't fade out the scrollbar when mouse is near.
+  if (mouse_is_near_scrollbar_)
+    StopAnimation();
 }
 
 void ScrollbarAnimationControllerThinning::DidMouseMoveNear(float distance) {

cc/input/scrollbar_animation_controller_thinning.h

@@ -35,6 +35,7 @@ class CC_EXPORT ScrollbarAnimationControllerThinning
   bool mouse_is_near_scrollbar() const { return mouse_is_near_scrollbar_; }
 
   void DidScrollUpdate(bool on_resize) override;
+  void DidScrollEnd() override;
 
   void DidMouseDown() override;
   void DidMouseUp() override;