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This CL adds a very basic landing page for the NUX onboarding experience.
To test this CL, add --enable-features=NuxOnboardingForceEnabled.

Current progress screenshot: https://imgur.com/a/2OBLbyq

There will be follow-up CLs to make it look more like the mocks, as well as
add other views for NUX.

Bug: 874142
Change-Id: Ie643def26d657fadfbbf556dd03df7cddf5976cc
Reviewed-on: https://chromium-review.googlesource.com/1235114
Commit-Queue: Scott Chen <scottchen@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: Hector Carmona <hcarmona@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593058}