Introduces a new scrollbar for message_center.

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 5b534d6c68cec9a0dcba5b74954fd3cbef434fd3..aa3800eb614cb579b262c0320d248a6e87d3a29e 100644
--- a/ui/views/controls/scrollbar/scroll_bar.h
+++ b/ui/views/controls/scrollbar/scroll_bar.h
@@ -86,6 +86,13 @@ class VIEWS_EXPORT ScrollBar : public View {
   // is the height for a horizontal scrollbar.
   virtual int GetLayoutSize() const = 0;
 
+  // Get the width or height for this scrollbar which overlaps with the content.
+  // Default is 0.
+  virtual int GetContentOverlapSize() const;
+
+  virtual void OnMouseEnteredScrollView(const ui::MouseEvent& event);
+  virtual void OnMouseExitedScrollView(const ui::MouseEvent& event);
+
  protected:
   // Create new scrollbar, either horizontal or vertical. These are protected
   // since you need to be creating either a NativeScrollBar or a