Updating old_generation_allocation_limit_ in Heap::NotifyContextDisposed
races with concurrent allocation. Fix race by making field atomic.

Bug: v8:11064, v8:10315
Change-Id: I030b7f90b2f5b344cdcea5185340dc54dc10298c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2521153
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71008}