Currently a TracedNode of a TracedReference is freed only if its target
V8 object is unreachable. This is problematic for TracedNodes created for
long-living (or immortal) V8 objects and leads to memory leaks.

This CL adds logic for collecting unreachable TracedNodes:
1) Each TracedNode gets a markbit. Initially the markbit is set (i.e.
   we have black allocation for TracedNodes).
2) During marking RegisterEmbedderReference sets the markbit of the
   corresonding TracedNode.
3) In the atomic pause of Mark-Compact when TracedNodes are iterated,
   we check the markbits and free TracedNodes with cleared markbits.
   After this processing all markbits are cleared for the next GC.

Note that the new logic does not apply to TracedNode that have
callbacks and/or destructors.

Bug: chromium:1029738
Change-Id: I38e76a8b4a84170793998988b1a7962e40874428
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1948722
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65347}