Change the resizing behaviour on insert to match that of the hash map
in base. Specifically, resize when hitting 80% occupancy.

Locally, I measure a ~6% improvement in serialization time from this
change.

Change-Id: I3fe84de39b2337859fe75fa6b3848198b82071ae
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2448798
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70324}