am 245e0cdd: Implement tiles synchronization When a tiledPage is painted, we want...

* commit '245e0cdd9ea80915059080a2bedbb9ebb5cc646c':
  Implement tiles synchronization When a tiledPage is painted, we want to avoid updating the base layer. GLWebViewState and WebView.cpp are now refcounting the base layer, delaying updating the base layer we use to paint until we are done with a full tile paint.