We need a protecting RefPtr in MediaQueryList::stop()

m_listeners.clear() can drop the last reference to MediaQueryListener, so we need to protect it before calling m_listeners.clear().

BUG=395409
R=cbiesinger@chromium.org

Review URL: https://codereview.chromium.org/414623004

git-svn-id: svn://svn.chromium.org/blink/trunk@178732 bbb929c8-8fbe-4397-9dbb-9b2b20218538