Re-enable SecurityTest.CallocOverflow

The test now works after the fix to the Clang roll script in
r227545 and the Clang roll in r228318.

BUG=304125

Review URL: https://codereview.chromium.org/27211002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228508 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228508