Move Peter into the main OWNERS file.

He has plenty of support from blink core owners.

BUG=None
R=jamesr@chromium.org

Review URL: https://codereview.chromium.org/23876032

git-svn-id: svn://svn.chromium.org/blink/trunk@158330 bbb929c8-8fbe-4397-9dbb-9b2b20218538