[iOS] Add mutator and set Reader mode font family pref from options UI

This CL adds the ReaderModeOptionsMutator protocol so that the font
family button in Reader mode options can be used to set the font family
pref.

Bug: 409941529, 409941491
Change-Id: If5c8c434cba0d880d6f4c9e4d8c0474f44b5b574
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6695732
Commit-Queue: Nohemi Fernandez <fernandex@google.com>
Auto-Submit: Quentin Pubert <qpubert@google.com>
Reviewed-by: Nohemi Fernandez <fernandex@google.com>
Commit-Queue: Quentin Pubert <qpubert@google.com>
Cr-Commit-Position: refs/heads/main@{#1481574}