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This CL adds right/left shelf alignment support for UnifiedSystemTray.

TEST=PanelLayoutManagerTest.TouchHitTestPanel
(--enable-features=SystemTrayUnified)
BUG=847104

Change-Id: I69d2755dd18d40508c3183ae08a75eef732dcce8
Reviewed-on: https://chromium-review.googlesource.com/1086900Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#564907}