Fix a ChromeFrame crash which occurs in the background worker thread while dereferencing a NULL a...

Fix a ChromeFrame crash which occurs in the background worker thread while dereferencing a NULL automation client.
This crash occurs when the active document is in the process of shutting down while there are active background
requests still pending.

Fix is to ensure that the background thread has been stopped before returning from the UrlmonUrlRequestManager::StopAll
function which gets called during CF shutdown.

Fixes bug http://code.google.com/p/chromium/issues/detail?id=102393

BUG=102393
Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=107995

[robertshield, 2011-10-31 18:38:14]

lg, is there any way to write a quick test for this?