RuntimeEnabledFeatures::PreferNonCompositedScrolling

Extracted from pdr@'s https://chromium-review.googlesource.com/c/chromium/src/+/2209993/3.
For testing non-composited-scrolling without depending on volatile
composited-scrolling trigger rules based on style.

Change-Id: I173e517da6539ca42336704c0a20942ef1c1e12b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2213323


Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Reviewed-by: Philip Rogers <pdr@chromium.org>
Reviewed-by: Sam Fortiner <samfort@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#772053}