heap: HeapVector: Separate GCed from DISALLOW_NEW type [8/N]

We separate HeapVector into two separate types:
- HeapVector: For use on stack and in-object fields.
- GCedHeapVector: For dynamically allocating with
  MakeGarbageCollected().

Trivial test and heap-related changes.

Bug: 392817527
Change-Id: I468ab3f7e21fc08bb08459bb0b3db4e03e2581e2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6415693
Auto-Submit: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1440204}