Oilpan: Make sure that vtables for garbage collected mixin objects have
vtable entries for the trace method.

If we get garbage collection during construction of an object with
a virtual trace method we need to make sure that all vtables in
the inheritance hierarchy have entries for the trace method.
Otherwise, we can get out-of-bounds access of a super class vtable.

R=erik.corry@gmail.com, haraken@chromium.org, wibling@chromium.org
BUG=382557

Review URL: https://codereview.chromium.org/406523002

git-svn-id: svn://svn.chromium.org/blink/trunk@178431 bbb929c8-8fbe-4397-9dbb-9b2b20218538