Initialization of request objects requires asynchronous hops between the UI and
IO threads. In some cases the browser may be destroyed, the mojo connection may
be aborted, or the ProxyURLLoaderFactory object may be deleted while
initialization is still in progress. This change fixes crashes and adds unit
tests that try to reproduce these conditions.
To test: Run `ceftests --gtest_repeat=50
--gtest_filter=ResourceRequestHandlerTest.Basic*Abort*`
The optional |extra_info| parameter provides an opportunity to specify extra
information specific to the created browser that will be passed to
CefRenderProcessHandler::OnBrowserCreated() in the render process.