Oilpan: Disallow tracing Raw/Ref/OwnPtr fields.

We want to completely disallow tracing off-heap fields since tracing of such fields can not be correctly identified by conservative stack scanning. This CL disallows tracing off-heap smart-pointers and raw pointers. We still allow tracing of off-heap collections since the on-heap versions don't yet support backings with vtables.

R=erik.corry@gmail.com
BUG=

Review URL: https://codereview.chromium.org/361853005

git-svn-id: svn://svn.chromium.org/blink/trunk@177281 bbb929c8-8fbe-4397-9dbb-9b2b20218538