Add back support for /Gv now that gyp-win64 has VS2013

This was removed in the last minute in r1984 because of failures in the
gyp-win64 bot.

BUG=
R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/645223002

git-svn-id: http://gyp.googlecode.com/svn/trunk@2007 78cadc50-ecff-11dd-a971-7dbc132099af