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Also make both backends use the same directory for files, and the same name for
the index file. Using the index file each backend will detect if the index
belongs to it. Hence, easier to make sure that both cache trees on disk are
mutually exclusive.

BUG=229437


Review URL: https://chromiumcodereview.appspot.com/13813015

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@193391 0039d316-1c4b-4281-b951-d872f2087c98