[FileAPI] Replace default leveldb::Env with leveldb::MemEnv in tests.

Use in-memory LevelDB in tests that don't test LevelDB behavior itself.

BUG=331988
R=kinuko@chromium.org, nhiroki@chromium.org, sky@chromium.org

Review URL: https://codereview.chromium.org/145693005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247631 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247631