When the embedder integrates in V8's garbage collector the performance
of the atomic phase is sensitive to how much embedder memory is found
through marking the overall transitive closure.

Before this patch, V8 would help out tracing the embedder's heap when
making progress through tasks but not on allocations. In addition, V8
would complete the garbage collection when it has observed it's own
marking worklists as empty 3 times (*). This can create performance
cliffs when there's a lot of work still to be done on the embedder
side.

This patch adds helping steps on allocation that are proportional to
the bytes that V8 would otherwise process, guaranteeing some progress
as long as there's V8 allocations. This allows us to remove (*).

Potential Tradeoffs:
- More time spent in V8's garbage collection metrics as we slightly
  limit the chances for the embedder to mark objects through tasks.
- Prolonged V8.execute time (JS execution)
+ Faster progress
+ Less memory
+ Smaller atomic pause time

Change-Id: I160f063209f7e129b9c884206f833706b69dadc1
Bug: chromium:1044630
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2025371
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66091}