Add synthetic field trial for BFCache & Prerender on WebView

We intend to run experiments BFCache & Prerender on WebView that
are not controlled by Finch, but instead by the WebView embedder.
To make sure that we can still analyze Chrome-side metrics and
crash regressions, we should register a synthetic field trial
when the BFCache/Prerender setting updater is explicitly called,
which indicates that the WebView embedder is part of the
external experiment, and we can know which group
(enabled/disabled) the specific caller belongs to from the
setting value.

Bug: b/346222905
Change-Id: I795d838b10eb577b87daab57e66730644cad633e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5611411


Reviewed-by: Richard (Torne) Coles <torne@chromium.org>
Auto-Submit: Rakina Zata Amni <rakina@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Commit-Queue: Rakina Zata Amni <rakina@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1313098}