Deprecates EmbedderHeapTracer::NumberOfWrappersToTrace and replaces it
with EmbedderHeapTracer::IsTracingDone.

V8 only really cares about the final state (emptiness) here and
embedders may choose implementations that have a hard time determinining
exact size for their work queues.

Bug: chromium:843903
Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: I1e141c47771ef08aab7dbe204e8175cfee99cf92
Reviewed-on: https://chromium-review.googlesource.com/1127599
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54311}