Enable creation of offscreen contexts which own their backing surface.

third_party/WebKit/Source/modules/webgl/WebGLContextAttributeHelpers.cpp

 // 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.
 
 #include "modules/webgl/WebGLContextAttributeHelpers.h"
 
 #include "core/frame/Settings.h"
 
 namespace blink {
 
 WebGLContextAttributes toWebGLContextAttributes(
     const CanvasContextCreationAttributes& attrs) {
   WebGLContextAttributes result;
   result.setAlpha(attrs.alpha());
   result.setDepth(attrs.depth());
   result.setStencil(attrs.stencil());
   result.setAntialias(attrs.antialias());
   result.setPremultipliedAlpha(attrs.premultipliedAlpha());
   result.setPreserveDrawingBuffer(attrs.preserveDrawingBuffer());
   result.setFailIfMajorPerformanceCaveat(attrs.failIfMajorPerformanceCaveat());
   return result;
 }
 
 Platform::ContextAttributes toPlatformContextAttributes(
     const CanvasContextCreationAttributes& attrs,
     unsigned webGLVersion) {
   Platform::ContextAttributes result;
+  result.alpha = attrs.alpha();
+  result.depth = attrs.depth();
+  result.stencil = attrs.stencil();
+  result.antialias = attrs.antialias();
   result.failIfMajorPerformanceCaveat = attrs.failIfMajorPerformanceCaveat();
   result.webGLVersion = webGLVersion;
   return result;
 }
 
 }  // namespace blink