Keep expanded if mouse moves off of scrollbar while dragging

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 5f1f343f55a23756180784789ffa0184dc6c56db..d489d457c8b2ac7bbf5113d5c8d5f5c50890cc97 100644
--- a/cc/input/input_handler.h
+++ b/cc/input/input_handler.h
@@ -153,6 +153,8 @@ class CC_EXPORT InputHandler {
   virtual ScrollStatus FlingScrollBegin() = 0;
 
   virtual void MouseMoveAt(const gfx::Point& mouse_position) = 0;
+  virtual void MouseDownAt(const gfx::Point& mouse_position) = 0;

[weiliangc, 2016/09/29 21:37:33]

I don't see the mouse_position being used. Is it possible to rename to MouseDown
and not have mouse_position passed in?

+  virtual void MouseUp() = 0;
 
   // Stop scrolling the selected layer. Should only be called if ScrollBegin()
   // returned SCROLL_STARTED.