Add gab to installer OWNERS file.

BUG=Gab is not an OWNER.
TEST=Gab is an OWNER.


Review URL: https://chromiumcodereview.appspot.com/10855032

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@150424 0039d316-1c4b-4281-b951-d872f2087c98

chrome/installer/OWNERS

+gab@chromium.org
 grt@chromium.org
 robertshield@chromium.org
 tommi@chromium.org