Attempts to get the chrome_frame_tests going again on the chrome frame builde...

Attempts to get the chrome_frame_tests going again on the chrome frame builder. Fixes as below:-
1. Removed a DCHECK from 
   ChromeFrameAutomationClient::CleanupRequests which checks 
   if the request was actually deleted. This DCHECK is not 
   correct anymore as the request is stopped asynchronously.

2. We now stop the worker thread in WM_DESTROY as the 
   activex window needs to be valid for Urlmon requests
   to be released correctly. In some cases IE reuses the 
   ActiveX instance. So we need to create the worker
   thread on demand.

TBR=amit



Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=30007