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When DICE is enabled and Sync is not turned on, the user menu should display a
view that contains an illustration on top, a promo text and a "Turn on Sync"
button.
This CL adds this feature.

Mocks can be found in the bug report.

Additional changes:
 - Extend the user menu width to 288px when DICE is enabled.
 - Add unit test for a negative |between_child_spacing| in BoxLayout.

Bug: 786369
Change-Id: I816ee93c8b8105182c599e42ca10fd7e413fe62b
Reviewed-on: https://chromium-review.googlesource.com/793451
Commit-Queue: Thomas Tangl <tangltom@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Evan Stade <estade@chromium.org>
Reviewed-by: Trent Apted <tapted@chromium.org>
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521668}