Have MutationObserverRegistration deal with references instead of pointers

Have MutationObserverRegistration deal with references instead of pointers when
possible. This makes the code look safer.

R=adamk@chromium.org, adamk

Review URL: https://codereview.chromium.org/178473026

git-svn-id: svn://svn.chromium.org/blink/trunk@168415 bbb929c8-8fbe-4397-9dbb-9b2b20218538