This CL introduces
 - GarbageCollectedMixin
 - IsGarbageCollectedMixinType
 - HeapObjectHeaderFor (utility method to get correct header for mixins)

Bug: chromium:1056170
Change-Id: I48c2c02fa57487824130ea3620c975a0785075e4
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2130275
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66953}