Match CanvasGradient::addColorStop to the spec.

Source/core/html/canvas/CanvasGradient.idl

Index: Source/core/html/canvas/CanvasGradient.idl
diff --git a/Source/core/html/canvas/CanvasGradient.idl b/Source/core/html/canvas/CanvasGradient.idl
index 0e07d5ef40aed8bb8447501945864c0b2fd0394d..4d06b833396fb6d1c4c3e48caf6f3303cda94971 100644
--- a/Source/core/html/canvas/CanvasGradient.idl
+++ b/Source/core/html/canvas/CanvasGradient.idl
@@ -1,5 +1,6 @@
 /*
  * Copyright (C) 2006 Apple Computer, Inc.  All rights reserved.
+ * Copyright (C) 2013 Intel Corporation. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
@@ -25,8 +26,7 @@
 [
 ] interface CanvasGradient {
 
-    [RaisesException] void addColorStop([Default=Undefined] optional float offset, 
-                      [Default=Undefined] optional DOMString color);
+    [RaisesException] void addColorStop(float offset, DOMString color);
 
 };