Oilpan: use transition types for Node RefPtrs in core/css and child
list mutation code.

R=haraken@chromium.org, oilpan-reviews@chromium.org, sigbjornf@opera.com
BUG=357163

Review URL: https://codereview.chromium.org/310043002

git-svn-id: svn://svn.chromium.org/blink/trunk@175368 bbb929c8-8fbe-4397-9dbb-9b2b20218538