The capacity may be modified on one thread when growing the table while
being used in a DCHECK (to sanity-check a provided
ExternalPointerHandle) on another thread, resulting in TSan failures.
This CL turns these accesses into atomic accesses and adds a comment
explaining when the capacity value can be used reliably.

Bug: chromium:1352148
Change-Id: I0b86a47e16cfa14ff2d296e7e507e38a3fb5893c
Cq-Include-Trybots: luci.v8.try:v8_linux64_heap_sandbox_dbg_ng,v8_linux_arm64_sim_heap_sandbox_dbg_ng
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3826244
Commit-Queue: Samuel Groß <saelo@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82444}