The detached CppHeap allows for allocation without invoking garbage
collections.  Allocated bytes are reported on the first allocation
after the CppHeap has been attached to an Isolate.

States:
- Detached: Allow only allocation;
- Attached: Unified heap GCs;
- Termination GC: Require detached state;

Destruction:
- Heap::TearDown: Detach if attached;
- ~CppHeap: Detach if attached;

Bug: chromium:1056170
Change-Id: I95ce029f36a7f10392257080b6e23e13cc0fc7b8
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2672940
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72579}