On-heap hash maps in blink are limited to Member types and non-traceable
types. The only exception to that is TraceWrapperV8Reference. Thus
ephemerons can have non-gced traceable values. This values should not be
pushed to the marking worklist since we expect everything in the
worklist to be marked and not in construction (but these values don't
have an object header).
Instead, when getting a non-gced value we should immediately trace it.

This is only relevant to ephemerons. Any other case would go through
Trace(const T&) that dispatches to the TraceTrait.

Blink has 1 use case of HeahHashMap from WeakMember<ScriptWrappable> to
TraceWrapperV8Reference.

Bug: chromium:1056170
Change-Id: Ia8f341d6bb1fc8fd3655b2be66b7814896549d1e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2696648Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72763}