Unify the checks on whether the message center should be used

There's a bunch of checks in a few places now: let's have a single flag
in ui_features.gni to toggle control.

This CL also explicitly drops support for iOS. In practice iOS wasn't
using the message center anyway, so the few checks can just be deleted.

BUG=739386

Change-Id: I783bc9732f35f5a400410628fb90341141e2e03a
Reviewed-on: https://chromium-review.googlesource.com/559539
Reviewed-by: Nico Weber <[email protected]>
Reviewed-by: Yoshiki Iguchi <[email protected]>
Commit-Queue: Peter Beverloo <[email protected]>
Cr-Commit-Position: refs/heads/master@{#490481}
7 files changed