Updating external memory counters from
Isolate::AdjustAmountOfExternalAllocatedMemory races with concurrent
allocation.

Bug: v8:10315
Change-Id: I42c8cad41280d3e14eb6745cb268ff6cdd958c75
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2352776
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69378}