Fix race condition introduced in r242200 where AwContentsIoThreadClient is queried before a subframe's entry is added.

This happened because the entry was updated on the UI thread through WebContentsObserver::RenderFrameCreated. The race condition was that the IO thread could dispatch a resource request for that subframe before the map is updated.

The fix is to send a message from the renderer to the IO thread when a subframe is created so that the map is updated before any resouce requests arrive.

BUG=304341
R=mkosiba@chromium.org

Review URL: https://codereview.chromium.org/135443002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@244521 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 244521