Added plumbing for the availablechange event from Blink to WebPresentationClient.

public/platform/WebPresentationClient.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 WebPresentationClient_h
+#define WebPresentationClient_h
+
+namespace blink {
+
+class WebPresentationController;
+
+// The implementation the embedder has to provide for the Presentation API to work.
+class WebPresentationClient {
+public:
+    virtual ~WebPresentationClient() { }
+
+    // Passes the Blink-side delegate to the embedder.
+    virtual void setController(WebPresentationController*) = 0;
+
+    // Called when the frame attaches the first event listener to or removes the
+    // last event listener from the |availablechange| event.
+    virtual void updateAvailableChangeWatched(bool watched) = 0;
+};
+
+} // namespace blink
+
+#endif // WebPresentationClient_h