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This adds support for validating the device name that a user enters
during the Nearby Share onboarding flow. Validation happens as the user
types and also when they try to proceed to the next screen. If an
invalid name is detected, an error message will appear and the "Next"
button will be disabled.

Screenshots:
  https://screenshot.googleplex.com/3USR7ftg7QCwJhR.png
  https://screenshot.googleplex.com/6Cr6pedUxWnUKju.png

Bug: b:169582004
Change-Id: I07d4822e0ea099b30d23ed34a5f70f01940a3009
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2441150Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: James Vecore <vecore@google.com>
Commit-Queue: Michael Hansen <hansenmichael@google.com>
Cr-Commit-Position: refs/heads/master@{#813193}