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This change makes mru_window_tracker.cc a SessionObserver and
moves the initial focus logic there.

Bug: 665064
Change-Id: I5c125e0bbe62b9d2c8d809d519d3ccfeade76f8f
Reviewed-on: https://chromium-review.googlesource.com/1070854
Commit-Queue: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561886}