The (age, context) pair has to be added atomically in to the weak
array of detached contexts. Otherwise, GC may happen after insertion
of age and observe inconsistent state.

Bug: chromium:1016703
Change-Id: Icb20bed4359904b2d976986a236558542e314bbf
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1895573
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64820}