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- Creates an interface (delagate) which provides alternate UI behaviors
  for the Cellular Setup flows.
- Cellular setup UI is embedded in either OOBE or os-settings, the
  delegate implements functionality depending on where it is embedded.
- Creates a default implementation of delegate interface in os-settings
- Updates cellular_setup ui test and adds fake_cellular_setup_ delagate

screenshot: https://screenshot.googleplex.com/4qdJyUh4yx52iGa.png

Bug: 1093185
Change-Id: Ifd604c8bb57643520ee69f36378053b60e09c822
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2419631
Commit-Queue: Nnamdi Theodore Johnson-kanu <tjohnsonkanu@google.com>
Reviewed-by: Azeem Arshad <azeemarshad@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#810853}