Isolate::GetHeapStatistics uses PagedSpace::Available, which races
with allocating background threads.

Bug: v8:10315
Change-Id: I6e0dc37d90e0c7a3e3dd2b8bdb77f2ea82372c13
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2349294Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69337}