ThreadedScrollPreventRenderingStarvation: Add chrome://flags variations

This adds ThreadedScrollPreventRenderingStarvation variations to
chrome://flags that align with what we're planning to try in Finch,
specifically threshold_ms values of 66, 100, 200, and 333. Exposing
these options in chrome://flags will make local testing easier.

In local testing on a Pixel 7 using theverge.com, which has a lot of
congestion if scrolling during loading, all of these were effective in
mitigating rendering starvation issues (i.e. scrolling an empty black
page) except 333ms, although I'd still like to test that on a Canary
build and on Finch.

Note: this also fixes the field trial config, which was using the wrong
parameter name.

Bug: 40833407
Change-Id: I588c1a02c68372f2b00256f4b4936ee9384f4308
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5905604
Commit-Queue: Scott Haseley <shaseley@chromium.org>
Reviewed-by: Ian Vollick <vollick@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1363393}