- Adds a SharedArrayBuffersEnabled callback and uses it to
  enable/disable SABs per context. The feature flag is used
  if no callback is registered.

Bug: chromium:923807
Change-Id: I4d3472fcd79b158cb50dc98793aece6dbbb81d93
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2316901Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Bill Budge <bbudge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69057}