Disallow use of the UserGestureIndicator outside of the main thread.

Dedicated workers touch code paths that create UserGestureIndicators,
e.g. when creating DOM timers. However, there will never be a real user
gesture on a background thread, and it shouldn't interfere with the
state of the foreground thread, so we just make those
UserGestureIndicators no-ops

BUG=318249
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/71423002

git-svn-id: svn://svn.chromium.org/blink/trunk@161936 bbb929c8-8fbe-4397-9dbb-9b2b20218538