Introduce a method to release the rtc peer connection handler

BUG=337572
R=adamk@chromium.org, perkj@chromium.org

Review URL: https://codereview.chromium.org/287103005

git-svn-id: svn://svn.chromium.org/blink/trunk@174540 bbb929c8-8fbe-4397-9dbb-9b2b20218538