Revert of Forward input and compositing tasks to the Blink scheduler (patchset #21...

Revert of Forward input and compositing tasks to the Blink scheduler (patchset #21 id:400001 of https://codereview.chromium.org/363383002/)

Reason for revert:
suspecting it's causing the high rate of flakes in Instrumentation test ContentShellTest

BUG=411756

Original issue's description:
> Forward input and compositing tasks to the Blink scheduler
>
> Instead of posting input and compositor tasks directly to the
> main thread message loop, forward them to the Blink scheduler
> so they can be prioritized over other tasks.
>
> BUG=391005
>
> Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=290845
>
> Committed: https://chromium.googlesource.com/chromium/src/+/68bab3290b45a8610ad875d3c92530477c16a948

TBR=jamesr@chromium.org,alexclarke@chromium.org,picksi@chromium.org,eseidel@chromium.org,skyostil@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=391005

Review URL: https://codereview.chromium.org/548253002

Cr-Commit-Position: refs/heads/master@{#293686}