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This adds an Engagement.IsHigh.Changed bit to the SessionFinished UKM
that will be true if the IsHigh bit changed during the lifetime of the
session.

BUG=814351

Change-Id: I48b296301a59d7f92ce6fd8ac7750c41ed2bbecd
Reviewed-on: https://chromium-review.googlesource.com/964181
Commit-Queue: Becca Hughes <beccahughes@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Cr-Commit-Position: refs/heads/master@{#549242}