Oilpan: Replace RefPtr to Node in MutationObserver-related classes with Oilpan transition types.

BUG=357163

Review URL: https://codereview.chromium.org/313023004

git-svn-id: svn://svn.chromium.org/blink/trunk@175474 bbb929c8-8fbe-4397-9dbb-9b2b20218538