heap: HeapVector: Separate GCed from DISALLOW_NEW type [18/N]

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

More event-related changes. Basically, the APIs require passing
snapshots and always required a copy anyways.

Bug: 392817527
Change-Id: Ic2ad66b1945759afc4aa29836149b56d497f9d28
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6429606
Auto-Submit: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1442119}