[SyncFS] Make DriveBackendSyncTest expectation strict

BUG=331988
R=nhiroki@chromium.org

Review URL: https://codereview.chromium.org/138153004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247449 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247449