heap: HeapVector: Separate GCed from DISALLOW_NEW type [20/N]

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

DOM parts related changes. This changes PartList and PartNodeList to
GCHeapVector as most usages actually allocate the vectors with
MakeGarbageCollected().

Bug: 392817527
Change-Id: I17582f5233344c2f32aa6231eb9d15f366de6ac7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6434050
Reviewed-by: Mason Freed <masonf@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1443362}