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message_center::UiController and UiDelegate were used by both the platform
with only popup and with the message center and popup. This CL is the first
CL of refactoring to split them into two for simplification.
The current dependency:
 CrOS: UiController --UiDelegate--> NotificationTray/UnifiedSystemTray
 Old Win/Linux: UiController --UiDelegate--> PopupsOnlyUiDelegate
 Old Mac: UiController --UiDelegate--> MessageCenterBridge

The new dependency:
 CrOS: UiController --UiDelegate--> NotificationTray/UnifiedSystemTray
 Old Win/Linux: PopupsOnlyUiController(new) --::Delegate(new)--> PopupsOnlyUiDelegate
 Old MacMac: PopupsOnlyUiController(new) --::Delegate(new)--> MessageCenterBridge

I will move the existing UiController and UiDelegate to ash/ and refactor
them in following CLs.

Bug: 869278
Test: Notification works on Linux

Change-Id: I8ab077f0339cb55cf25633aa481509f1775d43dd
Reviewed-on: https://chromium-review.googlesource.com/1154617
Commit-Queue: Yoshiki Iguchi <yoshiki@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Reviewed-by: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579715}