Oilpan: add Element/Node Member fields to stack allocated objects.

Tidy up the definition of some stack objects and have them refer to
Element or Node references by way of Members rather than raw pointers.

R=tkent@chromium.org,haraken@chromium.org
BUG=357163

Review URL: https://codereview.chromium.org/319083004

git-svn-id: svn://svn.chromium.org/blink/trunk@175646 bbb929c8-8fbe-4397-9dbb-9b2b20218538