[FedCM] Record whether the UI is shown or not

This patch adds a new histogram Blink.FedCm.DidShowUI to record
whether the FedCM UI is shown or not when the API is completed.
Also it adds a corresponding ukm metric.

UKM collection review doc:
https://docs.google.com/document/d/1KOfotlnsMNILQUvBCQqAJCq7Yg6hGuoVVZEQhJ4WNtY

Bug: 430543304
Change-Id: Id9d0bfcabaf38b168835a13984b802569e5f2978
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6780675
Auto-Submit: Nicolás Peña <npm@chromium.org>
Reviewed-by: Sun Yueru <yrsun@chromium.org>
Reviewed-by: Christian Biesinger <cbiesinger@chromium.org>
Commit-Queue: Sun Yueru <yrsun@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1492528}