This CL implements GC in a shared heap. A shared GC is started from
an attached client isolate that fails to allocate a shared object. In
order to perform a shared GC all other running client isolates need
to be stopped and their roots need to be scanned.

Bug: v8:11708
Change-Id: I45ac50e6b4a1e9270f9e39b69f9b8ee5e6e14134
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2964816Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Mythri Alle <mythria@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75606}