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Create a secondary file thread, FILE_USER_BLOCKING, for tasks requiring file IO which have user waiting for the results. Process appcache background tasks on that thread.

BUG=68894,78359
Review URL: http://codereview.chromium.org/8366020

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@113317 0039d316-1c4b-4281-b951-d872f2087c98