Oilpan: move custom element objects to the heap.

R=ager@chromium.org,haraken@chromium.org,dominicc@chromium.org
BUG=357163

Review URL: https://codereview.chromium.org/296703009

git-svn-id: svn://svn.chromium.org/blink/trunk@174948 bbb929c8-8fbe-4397-9dbb-9b2b20218538