Suppress IO-on-wrong-thread DCHECK during file downloads.

BUG=72740
TEST=Downloading a file in linux Debug build should not die with a DCHECK 
failure.

Review URL: http://codereview.chromium.org/6473004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@74856 0039d316-1c4b-4281-b951-d872f2087c98