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

cc/input/scrollbar_animation_controller_thinning.h

Index: cc/input/scrollbar_animation_controller_thinning.h
diff --git a/cc/input/scrollbar_animation_controller_thinning.h b/cc/input/scrollbar_animation_controller_thinning.h
index d1d4ee0bac8539ade7052c282b7914c4e863fb0e..86472977315578a235d1999c4cabfdb27098b594 100644
--- a/cc/input/scrollbar_animation_controller_thinning.h
+++ b/cc/input/scrollbar_animation_controller_thinning.h
@@ -24,7 +24,8 @@ class CC_EXPORT ScrollbarAnimationControllerThinning
       ScrollbarAnimationControllerClient* client,
       base::TimeDelta delay_before_starting,
       base::TimeDelta resize_delay_before_starting,
-      base::TimeDelta duration);
+      base::TimeDelta fade_duration,
+      base::TimeDelta thinning_duration);
 
   ~ScrollbarAnimationControllerThinning() override;
 
@@ -47,34 +48,41 @@ class CC_EXPORT ScrollbarAnimationControllerThinning
       ScrollbarAnimationControllerClient* client,
       base::TimeDelta delay_before_starting,
       base::TimeDelta resize_delay_before_starting,
-      base::TimeDelta duration);
+      base::TimeDelta fade_duration,
+      base::TimeDelta thinning_duration);
 
   void RunAnimationFrame(float progress) override;
 
  private:
-  // Describes whether the current animation should INCREASE (darken / thicken)
-  // a bar or DECREASE it (lighten / thin).
+  // Describes whether the current animation should INCREASE (thicken)
+  // a bar or DECREASE it (thin).
   enum AnimationChange { NONE, INCREASE, DECREASE };
-  float OpacityAtAnimationProgress(float progress);
-  float ThumbThicknessScaleAtAnimationProgress(float progress);
+  enum AnimatingProperty { OPACITY, THICKNESS };
+  float ThumbThicknessScaleAt(float progress);
   float AdjustScale(float new_value,
                     float current_value,
                     AnimationChange animation_change,
                     float min_value,
                     float max_value);
-  void ApplyOpacityAndThumbThicknessScale(float opacity,
-                                          float thumb_thickness_scale);
+  void ApplyOpacity(float opacity);
+  void ApplyThumbThicknessScale(float thumb_thickness_scale);
 
+  void SetCurrentAnimatingProperty(AnimatingProperty property);
+
+  bool is_visible_;

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

Hmm, I don't generally like "visible" booleans due to their many possible subtle
meanings.  Could you just store "float opacity" and check "if (opacity == 0.0f)"
in the various places?

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

Done.

   bool captured_;
   bool mouse_is_over_scrollbar_;
   bool mouse_is_near_scrollbar_;
   // Are we narrowing or thickening the bars.
   AnimationChange thickness_change_;
-  // Are we darkening or lightening the bars.
-  AnimationChange opacity_change_;
   // How close should the mouse be to the scrollbar before we thicken it.
   float mouse_move_distance_to_trigger_animation_;
 
+  base::TimeDelta fade_duration_;
+  base::TimeDelta thinning_duration_;
+
+  AnimatingProperty current_animating_property_;
+
   DISALLOW_COPY_AND_ASSIGN(ScrollbarAnimationControllerThinning);
 };