Oilpan: fix usages of GC derived types as part objects.

The use of a GC derived type can potentially lead to memory corruptions if ever an object of that type is held by, eg, Member. After the next Blink GC plugin roll the compiler will issuing errors for such usage.

R=ager@chromium.org
BUG=

Review URL: https://codereview.chromium.org/298623005

git-svn-id: svn://svn.chromium.org/blink/trunk@174379 bbb929c8-8fbe-4397-9dbb-9b2b20218538