Revert of [platform] Change default platform max thread pool size 4 -> 8 (patchset #1 id:1 of https://codereview.chromium.org/1782833006/ )

Reason for revert:
Seems to trigger bugs, needs investigation.

Original issue's description:
> [platform] Change default platform max thread pool size 4 -> 8
>
> BUG=
>
> Committed: https://crrev.com/ddc0f9982057e76dd4dc35b6ca80d2322adc9141
> Cr-Commit-Position: refs/heads/master@{#34711}

TBR=hpayer@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=

Review URL: https://codereview.chromium.org/1780353003

Cr-Commit-Position: refs/heads/master@{#34714}