Shift "commit point" for when a download will no longer accept cancels.

This CL shifts the commit point for a download to just after the download 
file release has been dispatched.  The download remains IN_PROGRESS as 
far as the outside world is concerned, but at this point it is committed to 
continue to completion.

BUG=123998
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[email protected]


Review URL: https://chromiumcodereview.appspot.com/10950015

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@158298 0039d316-1c4b-4281-b951-d872f2087c98
20 files changed