drive: Remove cache entry before metadata entry

Cache entry cannot be removed when mounted.
This check should be performed before removing metadata entry.

BUG=349380

Review URL: https://codereview.chromium.org/187893006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255656 0039d316-1c4b-4281-b951-d872f2087c98
1 file changed