Bug 187121

While investigating Bug 187008 I found that some WebKitLegacy clients trigger recursive loads while cancelling the loading of web content into a WebView. This has the following impacts: 1. FrameLoader::continueLoadAfterNavigationPolicy gets entered with a nullptr Policy Document Loader as well as a nullptr Provisional Document Loader. If we continue in this state, we hit a ton of assertions, and eventually crash with a nullptr exception. If we return early, the cancel and alternate page load complete properly. 2. WebFrameLoaderClient::dispatchDidStartProvisionalLoad can be re-entered which triggers a set of assertions and eventually a nullptr dereference. If we keep track of whether we have started a load on the current client object, and return early in those cases, the cancel and alternate page load complete properly.