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Previously, the setup flow used fake data and did not actually perform
setup. This CL integrates the WebUI dialog by:
(1) Adding Mojo bindings to the dialog's WebUIController.
(2) Adds externs for Mojo types to the Closure compiler pass.
(3) Updates WebUI code to use Mojo JS bindings.

Bug: 824568
Cq-Include-Trybots: luci.chromium.try:closure_compilation
Change-Id: I53acbc227b9d965f9337fbbdb84c90e285665745
Reviewed-on: https://chromium-review.googlesource.com/1139222
Commit-Queue: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578887}