Simplify task_manager code now that ProcessHostOnUI is the only path.

Bug: 904556
Change-Id: Ib8a2407f33ef7ac735148aa09f2f5e630ef28758
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3171458


Auto-Submit: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Ahmed Fakhry <afakhry@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/main@{#923645}