[ios] Add UKM metric for Find in Page actions

This metric will track which URLs users are making Find in Page search requests in addition to
whether or not matches were found. Having this metric will help with testing and measuring
effects of changes to Find in Page logic.

UKM review doc: https://docs.google.com/document/d/1qb4aZo1CAgSqo6OyBNAc1krAOQgNqYnrm6lDlwfbcZk/edit?usp=sharing

Bug: 896867
Change-Id: Ie0f78da7b586ec2292405fa75eca1f4cf0efe3b9
Reviewed-on: https://chromium-review.googlesource.com/c/1289696
Commit-Queue: Chris Lu <thegreenfrog@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Reviewed-by: Mike Dougherty <michaeldo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602812}
5 files changed