Route Scroll events through a cc::InputHandler

ui/views/controls/scrollbar/scroll_bar.h

Index: ui/views/controls/scrollbar/scroll_bar.h
diff --git a/ui/views/controls/scrollbar/scroll_bar.h b/ui/views/controls/scrollbar/scroll_bar.h
index acda1430ff1bd832645f26461f409ed3833c9c9d..f9c07f2f7db90e3b8ed1b9d0c5759575200646e2 100644
--- a/ui/views/controls/scrollbar/scroll_bar.h
+++ b/ui/views/controls/scrollbar/scroll_bar.h
@@ -43,6 +43,10 @@ class VIEWS_EXPORT ScrollBarController {
   virtual int GetScrollIncrement(ScrollBar* source,
                                  bool is_page,
                                  bool is_positive) = 0;
+
+  // Invoked by the scroll bar when it receives a ui::ScrollEvent. Typically
+  // these should be treated the same as events received over the viewport.
+  virtual void OnScrollEventFromScrollBar(ui::ScrollEvent* event) = 0;
 };
 
 /////////////////////////////////////////////////////////////////////////////