drive: Record number of cache files recovered after DB corruption

If large numbers are reported for this histogram, it should be difficult for users to find necessary files from the recovered directory.

BUG=277333
R=asvitkine@chromium.org, kinaba@chromium.org

Review URL: https://codereview.chromium.org/54193002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232298 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 232298