Make presumbit warnings ignore symbols that start with UNIT_TEST

UNIT_TEST_MODE was being flagged as use of UNIT_TEST on a cc file.

NOTRY=true
R=joi@chromium.org
BUG=

Review URL: https://codereview.chromium.org/60573010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235987 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235987