Nux Onboarding: add testing for signin-view

Bug: 881937
Change-Id: I01c66ec9463db1c76f8f45579be7cd210c780b8a
Reviewed-on: https://chromium-review.googlesource.com/c/1338475
Commit-Queue: Scott Chen <scottchen@chromium.org>
Reviewed-by: Hector Carmona <hcarmona@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609078}
5 files changed