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These view controllers are currently contained children of MainViewController.
When the experiment is enabled, they are presented instead.

This CL also adds a completion handler to [MainViewController
setActiveViewController:completion:].  This handler will be used in a future CL.

BUG=768563

Change-Id: Iefead6de0a1471dcd07c67d634acbab15c70b9c3
Reviewed-on: https://chromium-review.googlesource.com/683196Reviewed-by: edchin <edchin@chromium.org>
Commit-Queue: Rohit Rao (ping after 24h) <rohitrao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#504451}