Call setNeedsRedraw when scrollbar opacity changes.

cc/input/scrollbar_animation_controller.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/input/scrollbar_animation_controller.h"
 
 #include <algorithm>
 
 #include "base/time/time.h"
 #include "cc/trees/layer_tree_impl.h"
@@ skipping 152 matching lines @@
 
 float ScrollbarAnimationController::AnimationProgressAtTime(
     base::TimeTicks now) {
   base::TimeDelta delta = now - last_awaken_time_;
   float progress = delta.InSecondsF() / fade_out_duration_.InSecondsF();
   return std::max(std::min(progress, 1.f), 0.f);
 }
 
 void ScrollbarAnimationController::RunAnimationFrame(float progress) {
   ApplyOpacityToScrollbars(1.f - progress);
-  client_->SetNeedsRedrawForScrollbarAnimation();
   if (progress == 1.f)
     StopAnimation();
 }
 
 void ScrollbarAnimationController::DidScrollBegin() {
   currently_scrolling_ = true;
 }
 
 void ScrollbarAnimationController::DidScrollEnd() {
   bool has_scrolled = scroll_gesture_has_scrolled_;
@@ skipping 186 matching lines @@
       property_trees->effect_tree.OnOpacityAnimated(
           effective_opacity,
           property_trees->layer_id_to_effect_node_index[scrollbar->id()],
           scrollbar->layer_tree_impl());
     }
   }
 
   bool previouslyVisible = opacity_ > 0.0f;
   bool currentlyVisible = opacity > 0.0f;
 
+  if (opacity_ != opacity)
+    client_->SetNeedsRedrawForScrollbarAnimation();
+
   opacity_ = opacity;
 
   if (previouslyVisible != currentlyVisible)
     client_->DidChangeScrollbarVisibility();
 }
 
 }  // namespace cc