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..cd0e2f164472e82ebd3fee4e0fe2c0e242fa3ae8 100644
--- a/cc/input/scrollbar_animation_controller.h
+++ b/cc/input/scrollbar_animation_controller.h
@@ -45,7 +45,7 @@ class CC_EXPORT ScrollbarAnimationController {
       ScrollbarAnimationControllerClient* client,
       base::TimeDelta fade_out_delay,
       base::TimeDelta fade_out_resize_delay,
-      base::TimeDelta fade_out_duration);
+      base::TimeDelta fade_duration);
 
   // ScrollbarAnimationController for Desktop Overlay Scrollbar. It has show &
   // fade out animation and thinning animation.
@@ -53,10 +53,10 @@ class CC_EXPORT ScrollbarAnimationController {
   CreateScrollbarAnimationControllerAuraOverlay(
       int scroll_layer_id,
       ScrollbarAnimationControllerClient* client,
-      base::TimeDelta show_delay,
+      base::TimeDelta fade_in_delay,

[Evan Stade, 2017/04/20 14:33:45]

I had envisioned you combining these as well, but I'll let code owners provide
their preferences on that.

[chaopeng, 2017/04/20 14:50:38]

Now we have fade_in_delay, fade_out_delay and fade_out_resize_delay. They are
same in Aura, but fade_out_delay and fade_out_resize_delay are different in
Android.
https://cs.chromium.org/chromium/src/cc/input/scrollbar_animation_controller_...

Also we use different delay in test to recognize which task we post. I prefer to
keep it. weiliangc@ WDYT?

       base::TimeDelta fade_out_delay,
       base::TimeDelta fade_out_resize_delay,
-      base::TimeDelta fade_out_duration,
+      base::TimeDelta fade_duration,
       base::TimeDelta thinning_duration);
 
   ~ScrollbarAnimationController();
@@ -93,21 +93,24 @@ 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:
+  // Describes whether the current animation should FadeIn or FadeOut.
+  enum AnimationChange { NONE, FADE_IN, FADE_OUT };
+
   ScrollbarAnimationController(int scroll_layer_id,
                                ScrollbarAnimationControllerClient* client,
                                base::TimeDelta fade_out_delay,
                                base::TimeDelta fade_out_resize_delay,
-                               base::TimeDelta fade_out_duration);
+                               base::TimeDelta fade_duration);
 
   ScrollbarAnimationController(int scroll_layer_id,
                                ScrollbarAnimationControllerClient* client,
-                               base::TimeDelta show_delay,
+                               base::TimeDelta fade_in_delay,
                                base::TimeDelta fade_out_delay,
                                base::TimeDelta fade_out_resize_delay,
-                               base::TimeDelta fade_out_duration,
+                               base::TimeDelta fade_duration,
                                base::TimeDelta thinning_duration);
 
   ScrollbarSet Scrollbars() const;
@@ -124,8 +127,7 @@ class CC_EXPORT ScrollbarAnimationController {
 
   void Show();
 
-  void PostDelayedShow();
-  void PostDelayedFadeOut(bool on_resize);
+  void PostDelayedAnimation(AnimationChange animation_change, bool on_resize);
 
   bool Captured() const;
 
@@ -138,25 +140,26 @@ class CC_EXPORT ScrollbarAnimationController {
 
   base::TimeTicks last_awaken_time_;
 
-  // show_delay_ is only for the case where the mouse hovers near the screen
+  // fade_in_delay_ is only for the case where the mouse hovers near the screen
   // edge.
-  base::TimeDelta show_delay_;
+  base::TimeDelta fade_in_delay_;
   base::TimeDelta fade_out_delay_;
   base::TimeDelta fade_out_resize_delay_;
 
+  base::TimeDelta fade_duration_;
+
   bool need_trigger_scrollbar_show_;
 
   bool is_animating_;
+  AnimationChange animation_change_;
 
   const int scroll_layer_id_;
   bool currently_scrolling_;
   bool show_in_fast_scroll_;
 
-  base::CancelableClosure delayed_scrollbar_show_;
-  base::CancelableClosure delayed_scrollbar_fade_out_;
+  base::CancelableClosure delayed_scrollbar_animation_;
 
   float opacity_;
-  base::TimeDelta fade_out_duration_;
 
   const bool show_scrollbars_on_scroll_gesture_;
   const bool need_thinning_animation_;