Use dictionaries for context creation attributes. Eliminate custom bindings.

Source/core/html/canvas/CanvasContextCreationAttributes.idl

Index: Source/core/html/canvas/CanvasContextCreationAttributes.idl
diff --git a/Source/core/html/canvas/CanvasContextCreationAttributes.idl b/Source/core/html/canvas/CanvasContextCreationAttributes.idl
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+++ b/Source/core/html/canvas/CanvasContextCreationAttributes.idl
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+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// The spec for HTMLCanvasElement.getContext() defines the context
+// creation attributes as type "any". In order to eliminate custom
+// bindings for getContext(), we define a dictionary that contains the
+// union of all of the context types' attributes. Note that it is not
+// possible to use a union type for this purpose because two dictionary
+// types are not distinguishable.
+//
+// Fortunately, there aren't any context creation attributes which are
+// defined with different default values in different context
+// specifications. (The "alpha" value, in particular, has a default
+// value of true for both the Canvas2D and WebGL specifications.)
+//
+// N.B.: Web IDL doesn't support multiple inheritance of dictionaries.
+
+dictionary CanvasContextCreationAttributes {
+    // Canvas 2D attributes
+    boolean alpha = true;  // Also used for WebGL.
+    // Note: the bindings generator ignores the RuntimeEnabled
+    // extended attribute for dictionary members.
+    [RuntimeEnabled=ExperimentalCanvasFeatures] DOMString storage;
+
+    // WebGL attributes
+    boolean depth = true;
+    boolean stencil = false;
+    boolean antialias = true;
+    boolean premultipliedAlpha = true;
+    boolean preserveDrawingBuffer = false;
+    boolean failIfMajorPerformanceCaveat = false;
+};