Added plumbing for the availablechange event from Blink to WebPresentationClient.

public/platform/WebPresentationController.h

+// Copyright 2015 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 WebPresentationController_h
+#define WebPresentationController_h
+
+namespace blink {
+
+// The delegate Blink provides to WebPresentationClient in order to get updates.
+class WebPresentationController {
+public:
+    virtual ~WebPresentationController() { }
+
+    // Called when the presentation screen availability changes.
+    // May not be called if there's no registered listener for the event.
+    virtual void didChangeAvailability(bool available) = 0;
+
+    // Indicates if the frame has listeners to the |availablechange| event.
+    virtual bool isAvailableChangeWatched() const = 0;
+};
+
+} // namespace blink
+
+#endif // WebPresentationController_h