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 b3f219d621d47dc91e7e5b4e14971be466f5677c..8b987660c308f69a79a736761c6f055780e59651 100644
--- a/public/platform/Platform.h
+++ b/public/platform/Platform.h
@@ -559,7 +559,11 @@ public:
     //
     // May return null if GPU is not supported.
     // Returns newly allocated and initialized offscreen WebGraphicsContext3D instance.
+#ifdef ENABLE_EXPLICIT_GL_SHARE_GROUPS
+    virtual WebGraphicsContext3D* createOffscreenGraphicsContext3D(const WebGraphicsContext3D::Attributes&, WebGraphicsContext3D* share_context) { return 0; }
+#else
     virtual WebGraphicsContext3D* createOffscreenGraphicsContext3D(const WebGraphicsContext3D::Attributes&) { return 0; }
+#endif
 
     // Returns a newly allocated and initialized offscreen context provider. The provider may return a null
     // graphics context if GPU is not supported.