Share Group plumbing in Blink; Remove WebGL from default share group

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 540 matching lines @@
     // Enumeration histogram buckets are linear, boundaryValue should be larger than any possible sample value.
     virtual void histogramEnumeration(const char* name, int sample, int boundaryValue) { }
     // Unlike enumeration histograms, sparse histograms only allocate memory for non-empty buckets.
     virtual void histogramSparse(const char* name, int sample) { }
 
 
     // GPU ----------------------------------------------------------------
     //
     // May return null if GPU is not supported.
     // Returns newly allocated and initialized offscreen WebGraphicsContext3D instance.
+    // Passing an existing context to shareContext will create the new context in the same share group as the passed context.
+    virtual WebGraphicsContext3D* createOffscreenGraphicsContext3D(const WebGraphicsContext3D::Attributes&, WebGraphicsContext3D* shareContext) { return 0; }
     virtual WebGraphicsContext3D* createOffscreenGraphicsContext3D(const WebGraphicsContext3D::Attributes&) { return 0; }
 
     // Returns a newly allocated and initialized offscreen context provider. The provider may return a null
     // 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.
     //
@@ skipping 67 matching lines @@
     virtual WebDatabaseObserver* databaseObserver() { return 0; }
 
 
 protected:
     virtual ~Platform() { }
 };
 
 } // namespace blink
 
 #endif