[iOS] Add a metric for Lockdown Mode usage

This adds a metric tracking the fraction of main-frame navigation
actions that have Lockdown Mode enabled.

Bug: 1393127
Change-Id: Idd94db6ed675402f25acea13372cb13d790baf6a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4053300


Commit-Queue: Ali Juma <ajuma@chromium.org>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1075564}