[iOS Clean] Created OverlayService.

ios/clean/chrome/browser/ui/overlay_service/internal/overlay_queue_observer.h

Index: ios/clean/chrome/browser/ui/overlay_service/internal/overlay_queue_observer.h
diff --git a/ios/clean/chrome/browser/ui/overlay_service/internal/overlay_queue_observer.h b/ios/clean/chrome/browser/ui/overlay_service/internal/overlay_queue_observer.h
new file mode 100644
index 0000000000000000000000000000000000000000..dd03b9b1b681ec311e92f85a8d0a06f31d2b9be0
--- /dev/null
+++ b/ios/clean/chrome/browser/ui/overlay_service/internal/overlay_queue_observer.h
@@ -0,0 +1,34 @@
+// 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_INTERNAL_OVERLAY_QUEUE_OBSERVER_H_
+#define IOS_CLEAN_CHROME_BROWSER_UI_OVERLAYS_INTERNAL_OVERLAY_QUEUE_OBSERVER_H_
+
+#include "base/macros.h"
+
+class OverlayQueue;
+
+// Interface for listening to events occurring to OverlayQueues.
+class OverlayQueueObserver {
+ public:
+  OverlayQueueObserver() = default;
+  virtual ~OverlayQueueObserver() = default;
+
+  // Called when an BrowserCoordinator is added to |queue|.
+  virtual void OverlayQueueDidAddOverlay(OverlayQueue* queue) {}
+
+  // Called when the overlay being presented by |queue| is about to be stopped
+  // so that a replacement overlay can be started.
+  virtual void OverlayQueueWillReplaceVisibleOverlay(OverlayQueue* queue) {}
+
+  // Called after the overlay being presented by |queue| is stopped.
+  virtual void OverlayQueueDidStopVisibleOverlay(OverlayQueue* queue) {}
+
+  // Called when the overlays in |queue| are cancelled.
+  virtual void OverlayQueueDidCancelOverlays(OverlayQueue* queue) {}
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(OverlayQueueObserver);
+};
+#endif  // IOS_CLEAN_CHROME_BROWSER_UI_OVERLAYS_INTERNAL_OVERLAY_QUEUE_OBSERVER_H_