Add OWNERS for the top-level BUILD.gn file.

Otherwise CQ will complain with:

** Presubmit ERRORS **
Missing LGTM from an OWNER for these files:
    BUILD.gn

R=brettw@chromium.org, darin@chromium.org
TBR=darin
NOTRY=true

Review URL: https://codereview.chromium.org/132893002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@244049 0039d316-1c4b-4281-b951-d872f2087c98

OWNERS

 darin@chromium.org
 ben@chromium.org
 per-file .gitignore=*
+per-file .gn=brettw@chromium.org
+per-file BUILD.gn=brettw@chromium.org
 per-file DEPS=*
 per-file AUTHORS=*
 per-file WATCHLISTS=*