The TraceTrait<T> checks whether T is a mixin to decide whether we can
use the fast (arithmetic) or slow (bitmap) method to look up the HoH.
Before this CL, the mixin application would also be considered as a
mixin because the marker is present, resulting in all cases going
through the object start bitmap.

The initial intention was to use the arithmetic for the mixin
applications as those inherit from GCed.

Bug: chromium:1056170
Change-Id: Ib0ba82a8f98e0481d2879ebacc1ca9bd9e675858
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2643395
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72262}