Remove now-unused m_query member in MediaQueryListListener

R=esprehn@chromium.org,yoav@yoav.ws

Review URL: https://codereview.chromium.org/536453004

git-svn-id: svn://svn.chromium.org/blink/trunk@181261 bbb929c8-8fbe-4397-9dbb-9b2b20218538