Position close buttons in bubbles based on a layout delegate constant.

ui/views/views_delegate.h

Index: ui/views/views_delegate.h
diff --git a/ui/views/views_delegate.h b/ui/views/views_delegate.h
index cbe34562bbe7024cf47130da99a1ee8d8e0f998b..c34751993693d773ab4b804bb61c93e5bca5f444 100644
--- a/ui/views/views_delegate.h
+++ b/ui/views/views_delegate.h
@@ -201,6 +201,10 @@ class VIEWS_EXPORT ViewsDelegate {
   // DialogClientView's content view.
   virtual gfx::Insets GetDialogButtonInsets() const;
 
+  // Returns the distance between a dialog's edge and the close button in the
+  // upper trailing corner.
+  virtual int GetDialogCloseButtonMargin() const;
+
   // Returns the spacing between a pair of related horizontal buttons, used for
   // dialog layout.
   virtual int GetDialogRelatedButtonHorizontalSpacing() const;