[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-Original-Commit-Position: refs/heads/master@{#602812}(cherry picked from commit e6e2df6817fe5982c79d732332e578e1817a1511)
Reviewed-on: https://chromium-review.googlesource.com/c/1309139
Reviewed-by: Chris Lu <thegreenfrog@chromium.org>
Cr-Commit-Position: refs/branch-heads/3578@{#423}
Cr-Branched-From: 4226ddf99103e493d7afb23a4c7902ee496108b6-refs/heads/master@{#599034}
5 files changed