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UnifiedSystemTray class will replace old SystemTray class. Therefore, we
have to remove all the references that rely on old SystemTray
implementation.

UnifiedSystemTray does not have help button (it will be replaced by one
in app launcher), so we remove HelpStep from first run tutorial.

Screenshot: http://screen/ZcfkBjBZT0x
Design doc: http://go/cros-qs-restyling

TEST=manual
BUG=837508,837502

Change-Id: I90b8eff7095b9e49cddd31398349b3c5db8728df
Reviewed-on: https://chromium-review.googlesource.com/1063529Reviewed-by: James Cook <jamescook@chromium.org>
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#560212}