Oilpan: reduce visibility of Visitor::trace(const RawPtr<T>&).

With Oilpan enabled, fail earlier and make it a compile-time error to
attempt to trace a RawPtr<>. (Previously it would only trigger an
assert at run-time.)

With Oilpan disabled, the method is needed to compile tracing methods
over RawPtr-using transition types.

R=
BUG=340522

Review URL: https://codereview.chromium.org/167443003

git-svn-id: svn://svn.chromium.org/blink/trunk@167267 bbb929c8-8fbe-4397-9dbb-9b2b20218538