Allow GC of the shared heap without any attached clients. This
CL also disables incremental marking for shared heaps for now.

Bug: v8:11708
Change-Id: I1eb47a42fe3ced0f23f679ecaae0c32e09eab461
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2886878Reviewed-by: Victor Gomes <victorgomes@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74511}