Code outside of BrowserParent should just get the LayersId from a getter
and not worry about RenderFrame.
Differential Revision: https://phabricator.services.mozilla.com/D33562
--HG--
extra : rebase_source : 63f9f9680a7cb16a18d9e56999e02a124aa63429
extra : source : e86839ca63260b09184755c98890fa8abf371530
extra : histedit_source : 34333f5f78ecf9b4f3e12c6175a6e81724a41fb2