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Current Popup_Page UKM is recorded when the web contents is destroyed,
this can lead to recording pop-up settings for source ids different
than the pop-up's opener source id.

This changelist also adds browsertests for logging of the UKM and
makes the ukm for Popup_Page singular as it should be.

BUG=1044601

Change-Id: I241a520b61bda691ce61ab4fdbac875f3397ac08
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2087975
Commit-Queue: Justin Miron <justinmiron@google.com>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Charlie Harrison <csharrison@chromium.org>
Cr-Commit-Position: refs/heads/master@{#751359}