Also, enable concurrent GetBackPointer().

Bug: v8:7790
Change-Id: If92cb7cdd9e6f5eb9a636764ca8abe2fc2c05ab2
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2650205
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72483}