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Enables lock scree apps for dev channel, and adds a flag that can
be used to enable the feature to chrome://flags:
 *  enable-lock-screen-apps enables lock screen apps

BUG=715781

Change-Id: I3a4638d16af1585b6c18445ee7f405e55b0c2db5
Reviewed-on: https://chromium-review.googlesource.com/576658Reviewed-by: Rahul Chaturvedi <rkc@chromium.org>
Reviewed-by: Achuith Bhandarkar <achuith@chromium.org>
Commit-Queue: Toni Barzic <tbarzic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#487945}