[iOS Clean] Created OverlayService.

ios/clean/chrome/browser/ui/overlay_service/browser_coordinator+overlay_support.h

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef IOS_CLEAN_CHROME_BROWSER_UI_OVERLAYS_WEB_OVERLAY_COORDINATOR_H_
+#define IOS_CLEAN_CHROME_BROWSER_UI_OVERLAYS_WEB_OVERLAY_COORDINATOR_H_
+
+#import <Foundation/Foundation.h>
+
+#import "ios/shared/chrome/browser/ui/coordinators/browser_coordinator.h"
+
+class OverlayQueue;
+class OverlayService;
+
+// Interface exposing functionality to support being displayed via
+// OverlayService.
+@interface BrowserCoordinator (OverlaySupport)
+
+// Returns the OverlayService associated with the receiver's BrowserState.
+@property(nonatomic, readonly) OverlayService* overlayService;
+
+// Whether this BrowserCoordinator supports overlaying.  Defaults to NO.  If NO,
+// it cannot be presented via OverlayService.  If YES, the coordinator is
+// expected to:
+// - call |-overlayWasStopped| from its |-stop| so that the OverlayService can

[marq (ping after 24h), 2017/06/23 10:42:01]

It seems like these things could be guaranteed in either BrowserCoordinator
itself or in an OverlayCoordinator subclass. There's been enough changes in this
design that I've lost track of what the arguments against a subclass were. 

+//   show the next overlay,
+// - implement the |overlayQueue| property.
+@property(nonatomic, readonly) BOOL supportsOverlaying;
+
+// The queue used to present this overlay.
+@property(nonatomic, assign) OverlayQueue* overlayQueue;
+
+// Starts overlaying this coordinator over |overlayParent|.  The receiver will
+// be added as a child of |overlayParent|.
+- (void)startOverlayingCoordinator:(BrowserCoordinator*)overlayParent;
+
+// Called when the overlay is stopped so that the OverlayService can be
+// notified.  This function also removes the receiver as a child from its parent
+// coordinator.
+- (void)overlayWasStopped;

[marq (ping after 24h), 2017/06/23 10:42:01]

yeah, I'd rather have an OverlayCoordinator class whose -stop handles notifying
the service and (separately) unparenting. 

+
+// Performs cleanup tasks for the overlay.  This allows for deterministic
+// cleanup to occur for coordinators whose UI has not been started. Rather than
+// relying on |-dealloc| to perform cleanup, |-cancelOverlay| can be used to
+// perform cleanup tasks deterministically.
+- (void)cancelOverlay;
+
+@end
+
+#endif  // IOS_CLEAN_CHROME_BROWSER_UI_OVERLAYS_WEB_OVERLAY_COORDINATOR_H_