[SyncFS] Add rest of DriveBackendSyncTest for conflict cases

BUG=331988

Review URL: https://codereview.chromium.org/158613003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250710 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250710