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

public/platform/Platform.h

Index: public/platform/Platform.h
diff --git a/public/platform/Platform.h b/public/platform/Platform.h
index 547bcec99fac10d47461bac44de4998fc21ed832..d981bd4240ff8d2a97307b44dcfd0090606ed0fc 100644
--- a/public/platform/Platform.h
+++ b/public/platform/Platform.h
@@ -558,6 +558,8 @@ public:
     //
     // 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