Minor improvements for on heap reference vectors:
- Use emplace_back instead of push_back.
- Reserve initial capacity.

Bug: v8:11993
Change-Id: Ie2f9ad9cc48d1b48a76419459a9ac721f4079175
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3124802
Commit-Queue: Victor Gomes <victorgomes@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#76538}