This is a reland of 7ef67b2e

Manually checked that the CL was not the culprit breaking
  media_blink_unittests --gtest_filter=WebMediaPlayerImplTest.MemDumpReporting

Original change's description:
> heap: Fix initial GC configuration for C++-only heaps
>
> 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/+/2944935
> Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
> Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#75012}

Bug: chromium:1217076
Change-Id: I482d8525379e33095834d5b41be8bb49bdd8a5d4
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2949094
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Auto-Submit: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75048}