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Also rename UserSelectionScreenProxy to UserBoardViewMojo.

The backend is a mojo client; the browser-side code should not know
that the implementation is in views.

Bug: 784495
Change-Id: I1f3933e7191a9be8f1d73720a27c747391763737
Reviewed-on: https://chromium-review.googlesource.com/1008843
Commit-Queue: Jacob Dufault <jdufault@chromium.org>
Reviewed-by: Achuith Bhandarkar <achuith@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550458}