[SyncFS] Use leveldb::MemEnv in test code

BUG=331988
R=nhiroki@chromium.org

Review URL: https://codereview.chromium.org/148183004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247447 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247447