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This adds a new boolean to the Popup.Closed UKM: WindowOpenDisposition.
This records the disposition used to create the popup, recorded
at the time the pop-up tracker is created.

This will be used to better understand how pop-ups are opened and
perceived by users in order to identify more abusive pop-up
experiences and improve the pop up blocker.

BUG=1044601

Change-Id: Ie32bff8f5753eb4210054569058e6df75d5f5258
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2075138
Commit-Queue: Justin Miron <justinmiron@google.com>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Ted Choc <tedchoc@chromium.org>
Reviewed-by: Charlie Harrison <csharrison@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#748252}