Enable creation of offscreen contexts which own their backing surface.

third_party/WebKit/public/platform/Platform.h

 /*
  * Copyright (C) 2012 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 441 matching lines @@
   // recordAction(UserMetricsAction("MyAction"))
   virtual void recordAction(const UserMetricsAction&) {}
 
   typedef uint64_t WebMemoryAllocatorDumpGuid;
 
   // GPU ----------------------------------------------------------------
   //
   struct ContextAttributes {
     bool failIfMajorPerformanceCaveat = false;
     unsigned webGLVersion = 0;
+    bool supportOwnOffscreenSurface = false;
+    // The remaining values are only used if supportOwnOffscreenSurface is true.
+    bool supportAlpha = true;
+    bool supportDepth = false;
+    bool supportAntialias = true;
+    bool supportStencil = false;
   };
   struct GraphicsInfo {
     unsigned vendorId = 0;
     unsigned deviceId = 0;
     unsigned processCrashCount = 0;
     unsigned resetNotificationStrategy = 0;
     bool sandboxed = false;
     bool amdSwitchable = false;
     bool optimus = false;
     WebString vendorInfo;
@@ skipping 199 matching lines @@
  protected:
   Platform();
   virtual ~Platform() {}
 
   WebThread* m_mainThread;
 };
 
 }  // namespace blink
 
 #endif