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 navigator.presentation to work.

[Peter Beverloo, 2015/01/07 19:53:11]

nit: s/navigator.presentation/the Presentation API/. The property it's exposed
on doesn't really matter (any may change if you ever decide to expose part of it
in workers).

[whywhat, 2015/01/08 16:05:28]

Done.

+class WebPresentationClient {
+public:
+    virtual ~WebPresentationClient() { }
+
+    // Passes the Blink-side delegate to the embedder.
+    virtual void setController(WebPresentationController*) = 0;
+
+    // Called when the frame attaches a new event listener to the |availablechange| event.
+    virtual void addDeviceAvailabilityListener() = 0;
+    // Called when the frame removes the event listener from the |availablechange| event.
+    virtual void removeDeviceAvailabilityListener() = 0;
+};
+
+} // namespace blink
+
+#endif // WebPresentationClient_h