Reland "Add fiedtrial testing config for Reader Mode feature on iOS."

This is a reland of commit 069978a0791ee0fccbfebceb2d4ed2576faf8e19

Original change's description:
> Add fiedtrial testing config for Reader Mode feature on iOS.
>
> Bug: 426175025
> Change-Id: I80f3f134ab1b0cc47ad5594d7eae66dc2b240f01
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6768685
> Reviewed-by: Quentin Pubert <qpubert@google.com>
> Auto-Submit: Nohemi Fernandez <fernandex@google.com>
> Commit-Queue: Quentin Pubert <qpubert@google.com>
> Cr-Commit-Position: refs/heads/main@{#1488879}

Bug: 426175025
Change-Id: I064199c8aa4c35abd9312282fc58b9a953c130e7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6771569
Reviewed-by: Quentin Pubert <qpubert@google.com>
Commit-Queue: Nohemi Fernandez <fernandex@google.com>
Cr-Commit-Position: refs/heads/main@{#1489514}