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This adds the back-end functionality of setting default for the NUX
set-default module, by subclassing a very similar handler from Settings.

There will be follow-up CLs to match front-end with mock.

Bug: 874122
Change-Id: I2e54ed7846e036cf48f4d6b22f2a7b867d65baa6
Reviewed-on: https://chromium-review.googlesource.com/1222914
Commit-Queue: Scott Chen <scottchen@chromium.org>
Reviewed-by: Hector Carmona <hcarmona@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595694}