Move ViewportListener to use RefPtr.

The viewportListeners in MediaQueryMatcher were stored with raw pointers for no good reason.
Even though there was no related bug that I'm aware of, that is bug-prone. This CL changes these references to RefPtr.

Review URL: https://codereview.chromium.org/530413002

git-svn-id: svn://svn.chromium.org/blink/trunk@181362 bbb929c8-8fbe-4397-9dbb-9b2b20218538