Heaps in V8 start with a large limit that is shrunk upon young
generation GCs, based on some liveness estimate. This provides best
throughput during startup while at the same time finding a reasonable
first limit.

For C++ (embedder memory) there is no estimate which is why it was
piggy-backing on V8. This breaks in scenarios where no JS memory is
allocated.

In this fix we start a memory reducer after embedder memory has hit
the activation threshold if no GC happened so far. As soon as a single
Scavenger has happened, we leave it up to the JS estimate to figure
out a limit. Memory reducing GCs will then find a regular limit based
on the initial live size.

Drive-by: Give embedders the same activiation threshold of 8MB as JS.

Bug: chromium:1217076
Change-Id: I8469696002ac2af8d75d6b47def062d2608387a1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2944935Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75012}