Revert "Increase FRE Source Trial percentages on Stable."

This reverts commit 17efb4a8181d21cbef5b2a3c7d82507b216633ea.

Reason for revert: There is a bug in the assignment of the groups

Original change's description:
> Increase FRE Source Trial percentages on Stable.
>
> The enabled and control groups for the FRE Source Trial on the Stable channel are increased from 1% to 20%, with the default group adjusted accordingly.
>
> The trial is only being run in iOS.
>
> https://uma.googleplex.com/p/chrome/variations?sid=6cb0b62348f1b04d33a1f4c66f711751
>
> DO NOT MERGE
>
> Bug: 407991309
> Change-Id: I63fee3ee74cdd9b5a51163f6c88ec80a1b0a332f
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6905454
> Reviewed-by: Caitlin Fischer <caitlinfischer@google.com>
> Commit-Queue: Ramon Cano Aparicio <rcanoaparicio@google.com>
> Cr-Commit-Position: refs/heads/main@{#1513029}

Bug: 407991309
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Change-Id: I088eed85981109042ceb3277f051160c27465ddf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6929947
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Ramon Cano Aparicio <rcanoaparicio@google.com>
Reviewed-by: Caitlin Fischer <caitlinfischer@google.com>
Cr-Commit-Position: refs/heads/main@{#1513077}
NOKEYCHECK=True
GitOrigin-RevId: 74d2920869420985fcef8bf251813ca153a10926