Sort and uniq sync_file_system entries in chrome_tests_unit.gypi

TBR=nhiroki@chromium.org

Review URL: https://codereview.chromium.org/96993003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237995 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237995