Oilpan: Fix conservative marking of objects with uninitialized vtables.

I used Visitor::markNoTracing but that is only implemented for
the object pointer and not for header pointers. This change
implements markNoTracing for header pointers and adds a regression
test.

R=oilpan-reviews@chromium.org, wibling@chromium.org, zerny@chromium.org
BUG=404511

Review URL: https://codereview.chromium.org/487683002

git-svn-id: svn://svn.chromium.org/blink/trunk@180528 bbb929c8-8fbe-4397-9dbb-9b2b20218538