Reland "Register a sticky synthetic trial when a readable page is loaded"

This is a reland of commit 31a9a18ccf58240f29275d55100f42335ea9eb02

Original change's description:
> Register a sticky synthetic trial when a readable page is loaded
>
> The trial name and group name depend on the field trial overriding the
> ReadAloud flag. If the ReadAloud trial is "ReadAloudLaunch", the
> synthetic trial name will be "ReadAloudLaunch_KnownReadable" and the
> group name will be the same.
>
> Low-Coverage-Reason: COVERAGE_UNDERREPORTED, see https://crbug.com/1515360 for details.
> Bug: b/316375886
> Change-Id: I9584c5cc9c57e8b61ec1b639b1bdfdb8953159ab
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5126973
> Reviewed-by: Basia Zimirska <basiaz@google.com>
> Commit-Queue: Ian Wells <iwells@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#1243007}

Bug: b/316375886
Change-Id: I6932b71530b2747878b19f205cd1f75d0240b202
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5170670
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Commit-Queue: Ian Wells <iwells@chromium.org>
Reviewed-by: Basia Zimirska <basiaz@google.com>
Cr-Commit-Position: refs/heads/main@{#1243147}