Don't fade in overlay scrollbar when user interacting the content under scrollbar.

cc/input/input_handler.h

Index: cc/input/input_handler.h
diff --git a/cc/input/input_handler.h b/cc/input/input_handler.h
index 4e68a3d86e0749481d2cd9c0b5ec019efd253802..47b56eb81055c9d2824a85894a41c7412f3c7979 100644
--- a/cc/input/input_handler.h
+++ b/cc/input/input_handler.h
@@ -158,7 +158,8 @@ class CC_EXPORT InputHandler {
   // SCROLL_IGNORED if not.
   virtual ScrollStatus FlingScrollBegin() = 0;
 
-  virtual void MouseMoveAt(const gfx::Point& mouse_position) = 0;
+  virtual void MouseMoveAt(const gfx::Point& mouse_position,
+                           bool mouse_down) = 0;

[bokan, 2017/06/08 14:02:38]

Can we just keep track of the mouse state from the ScrollbarAnimationController?
That is, we already get MouseDown and MouseUp, just keep track of the button
state using those. I realize it's slightly different behavior since if you start
a click in one scroller and drag outside of it other scrollers can fade-in their
scrollbars. But I think that's ok and not worth the added complexity. Those
faded in scrollbars wont prevent you from dragging/selecting under them. 

   virtual void MouseDown() = 0;
   virtual void MouseUp() = 0;
   virtual void MouseLeave() = 0;