b=622956; [webgl] add stub extension mechanism, fixes extension tests; r=bjacob

content/canvas/src/WebGLContextGL.cpp

@@ -4201,3 +4201,10 @@ int mozilla::GetWebGLTexelFormat(GLenum format, GLenum type)
         }
     }
 }
+
+NS_IMETHODIMP
+WebGLContext::GetExtension(const nsAString& aName, nsISupports **retval)
+{
+    *retval = nsnull;
+    return NS_OK;
+}

dom/interfaces/canvas/nsIDOMWebGLRenderingContext.idl

@@ -834,4 +834,7 @@ interface nsIDOMWebGLRenderingContext : nsISupports
   // Most useful for querying GL_VENDOR/GL_RENDERER for identifying
   // the underlying renderer to the user.
   [noscript] DOMString mozGetUnderlyingParamString(in WebGLenum pname);
+
+  // extensions
+  nsISupports getExtension(in DOMString name);
 };