change overlay scrollbar hover show to hover fade in

cc/input/scrollbar_animation_controller.h

Index: cc/input/scrollbar_animation_controller.h
diff --git a/cc/input/scrollbar_animation_controller.h b/cc/input/scrollbar_animation_controller.h
index d481eae15fbc6467ab1408c5b1252359e024c320..808b03a1af690fa4433cd338d2d6f0d53c55d918 100644
--- a/cc/input/scrollbar_animation_controller.h
+++ b/cc/input/scrollbar_animation_controller.h
@@ -56,6 +56,7 @@ class CC_EXPORT ScrollbarAnimationController {
       base::TimeDelta show_delay,
       base::TimeDelta fade_out_delay,
       base::TimeDelta fade_out_resize_delay,
+      base::TimeDelta fade_in_duration,
       base::TimeDelta fade_out_duration,
       base::TimeDelta thinning_duration);
 
@@ -93,7 +94,7 @@ class CC_EXPORT ScrollbarAnimationController {
   bool MouseIsNearScrollbar(ScrollbarOrientation orientation) const;
   bool MouseIsNearAnyScrollbar() const;
 
-  static constexpr float kMouseMoveDistanceToTriggerShow = 30.0f;
+  static constexpr float kMouseMoveDistanceToTriggerFadeIn = 30.0f;
 
  private:
   ScrollbarAnimationController(int scroll_layer_id,
@@ -107,6 +108,7 @@ class CC_EXPORT ScrollbarAnimationController {
                                base::TimeDelta show_delay,
                                base::TimeDelta fade_out_delay,
                                base::TimeDelta fade_out_resize_delay,
+                               base::TimeDelta fade_in_duration,
                                base::TimeDelta fade_out_duration,
                                base::TimeDelta thinning_duration);
 
@@ -124,7 +126,7 @@ class CC_EXPORT ScrollbarAnimationController {
 
   void Show();
 
-  void PostDelayedShow();
+  void PostDelayedFadeIn();
   void PostDelayedFadeOut(bool on_resize);
 
   bool Captured() const;
@@ -134,6 +136,9 @@ class CC_EXPORT ScrollbarAnimationController {
 
   void ApplyOpacityToScrollbars(float opacity);
 
+  // Describes whether the current animation should FadeIn or FadeOut.
+  enum AnimationChange { NONE, FADE_IN, FADE_OUT };
+
   ScrollbarAnimationControllerClient* client_;
 
   base::TimeTicks last_awaken_time_;
@@ -144,9 +149,13 @@ class CC_EXPORT ScrollbarAnimationController {
   base::TimeDelta fade_out_delay_;
   base::TimeDelta fade_out_resize_delay_;
 
+  base::TimeDelta fade_in_duration_;
+  base::TimeDelta fade_out_duration_;
+
   bool need_trigger_scrollbar_show_;
 
   bool is_animating_;
+  AnimationChange animation_change_;
 
   const int scroll_layer_id_;
   bool currently_scrolling_;
@@ -156,7 +165,6 @@ class CC_EXPORT ScrollbarAnimationController {
   base::CancelableClosure delayed_scrollbar_fade_out_;
 
   float opacity_;
-  base::TimeDelta fade_out_duration_;
 
   const bool show_scrollbars_on_scroll_gesture_;
   const bool need_thinning_animation_;