Prevent overlay scrollbars expand or hover together

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 bb3f810a4c2269d58267730ea97694916de2a899..b52b29bd4f137ea6e00686237e07351e0a64e266 100644
--- a/cc/input/scrollbar_animation_controller.h
+++ b/cc/input/scrollbar_animation_controller.h
@@ -5,10 +5,13 @@
 #ifndef CC_INPUT_SCROLLBAR_ANIMATION_CONTROLLER_H_
 #define CC_INPUT_SCROLLBAR_ANIMATION_CONTROLLER_H_
 
+#include <vector>
+
 #include "base/cancelable_callback.h"
 #include "base/memory/weak_ptr.h"
 #include "base/time/time.h"
 #include "cc/base/cc_export.h"
+#include "cc/input/scrollbar.h"
 #include "cc/layers/layer_impl.h"
 #include "cc/layers/scrollbar_layer_impl_base.h"
 #include "ui/gfx/geometry/vector2d_f.h"
@@ -16,6 +19,7 @@
 namespace cc {
 
 class ScrollbarAnimationController;
+using std::vector;
 
 class CC_EXPORT ScrollbarAnimationControllerClient {
  public:
@@ -35,9 +39,16 @@ class CC_EXPORT ScrollbarAnimationControllerClient {
 // specialized implementation.
 class CC_EXPORT ScrollbarAnimationController {
  public:
+  struct DistanceToScrollbar {
+    ScrollbarOrientation orientation_;
+    float distance_;
+
+    DistanceToScrollbar(ScrollbarOrientation orientation, float distance)
+        : orientation_(orientation), distance_(distance) {}
+  };
   virtual ~ScrollbarAnimationController();
 
-  bool Animate(base::TimeTicks now);
+  virtual bool Animate(base::TimeTicks now);
 
   virtual void DidScrollBegin();
   virtual void DidScrollUpdate(bool on_resize);
@@ -45,7 +56,7 @@ class CC_EXPORT ScrollbarAnimationController {
   virtual void DidMouseDown() {}
   virtual void DidMouseUp() {}
   virtual void DidMouseLeave() {}
-  virtual void DidMouseMoveNear(float distance) {}
+  virtual void DidMouseMoveNear(vector<DistanceToScrollbar>) {}

[bokan, 2016/12/06 19:44:46]

Rather than introducing DistanceToScrollbar, I think using (Orientation,
distance) as params here is better. If you need to just make two calls at the
call site.

   virtual bool ScrollbarsHidden() const;
 
  protected: