Disk cache: Switch the disk cache to use the cache_thread.

Add an InFlightBackendIO class that handles posting of
cacheoperations back and forth between the IO thread and
the cachethread.

BUG=26730
TEST=unit tests

Review URL: http://codereview.chromium.org/2945002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@52185 0039d316-1c4b-4281-b951-d872f2087c98
21 files changed