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This allows the settings page to request that multi-device features be
enabled or disabled via clicks on a toggle.

Bug: 824568
Change-Id: I7edf3991036d5119a1bed8cf67f82b2138594b13
Reviewed-on: https://chromium-review.googlesource.com/1176870
Commit-Queue: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: Tommy Li <tommycli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583839}