JS Bindings: Fold generated interface.DelegatingStubClass into interface.StubClass

The Stub class now delegates to its $delegate property, if there is one. Otherwise it behaves as before.

Method dispatch looks like this:

      if (this.delegate$) {
         if (this.delegate$.createURLLoader)
           this.delegate$.createURLLoader(params.loader);
      } else {
           this.createURLLoader(params.loader);
      }

BUG=433485
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/736793003