Remove dead code related to compositing from Sky

sky/engine/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 327 matching lines @@
     // graphics context if GPU is not supported.
     virtual WebGraphicsContext3DProvider* createSharedOffscreenGraphicsContext3DProvider() { return 0; }
 
     // Returns true if the platform is capable of producing an offscreen context suitable for accelerating 2d canvas.
     // This will return false if the platform cannot promise that contexts will be preserved across operations like
     // locking the screen or if the platform cannot provide a context with suitable performance characteristics.
     //
     // This value must be checked again after a context loss event as the platform's capabilities may have changed.
     virtual bool canAccelerate2dCanvas() { return false; }
 
-    virtual bool isThreadedCompositingEnabled() { return false; }
-
 protected:
     virtual ~Platform() { }
 };
 
 } // namespace blink
 
 #endif  // SKY_ENGINE_PUBLIC_PLATFORM_PLATFORM_H_