[blink scheduler] Update kThreadedScrollPreventRenderingStarvation feature

Compositor task queues (which run BeginMainFrame) have low priority
during compositor scrolling, which can lead to rendering starvation,
causing checker-boarding or scrolling stale content.

Previously we experimented with mitigating these issues with a few
options:
 1. low priority w/ delay-based anti-starvation
 2. normal priority w/ delay-based anti-starvation
 3. very high priority

(1) was promising, but more experimentation is needed. The others were
probably a bit too aggressive, negatively affecting smoothness.

This CL updates the previous experiment by:
 - Removing options (2) and (3) above.

 - Making the anti-starvation delay configurable, instead of using the
   existing 100ms delay (used when not doing CC scrolling). This is
   controlled by the "threshold_ms" parameter.

 - Integrating with the newly added starved-by-render-blocking-tasks
   anti-starvation policy, which elevates the CC TQ priority to
   "extremely high" if 500ms of render-blocking tasks have run since the
   last frame. This starvation will kick in during CC scrolling with
   this feature (it does not by default) -- but not until the
   configurable threshold is reached.

Bug: 40833407
Change-Id: I37ce697700574d5960115541012ac73becc7dd30
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5855734
Reviewed-by: Philip Rogers <pdr@chromium.org>
Commit-Queue: Scott Haseley <shaseley@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1356100}