This can be useful when there may be multiple callbacks attached by
code that's not directly tied to a single isolate, e.g. working
on a per-context basis.

This also allows rephrasing the global non-isolate APIs in terms
of this new API, rather than working around it inside `src/heap`.

TBR=hpayer@chromium.org

Bug: 
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: I2e490ec40d1a34ea812f25f41ef9741d2116d965
Reviewed-on: https://chromium-review.googlesource.com/647548Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47923}