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This CL ports unit tests from TrayTilesTest.
Also this CL fixes DCHECK failures and crash in logged out state.

UnifiedSystemTray design doc: go/cros-qs-restyling

TEST=TopShortcutsViewTest(ash_unittests)
BUG=none

Change-Id: Id32540a3213866aa86b89025f1aa7bc97bae989b
Reviewed-on: https://chromium-review.googlesource.com/958304Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543031}