Re-enable GN as part of GYP execution.

BUG=288991
R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/104033004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239254 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239254