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This adds DefaultView.VisibleRows metrics to UnifiedSystemTray. It was
previously implemented in old SystemTray.

PatchSet 1: Moving SystemTrayItem::UmaType to SystemTrayItemUmaType
PatchSet 2: Implement the metrics in UnifiedSystemTray

TEST=manual
BUG=842079

Change-Id: Ied65e288147c59b6df0a8312a86e30a35fa4b953
Reviewed-on: https://chromium-review.googlesource.com/1094821
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567108}