Addressing TODOs in the code to support size_t indices everywhere.

Bug: v8:4153
Change-Id: I06432293799feed3b6a0c634cbbdcac250430d19
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1962269Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65680}