[PresentationAPI] Added/changed public/platform interfaces for start/joinSession

public/platform/modules/presentation/WebPresentationSessionClient.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 WebPresentationSessionClient_h
+#define WebPresentationSessionClient_h
+
+#include "public/platform/WebString.h"
+
+namespace blink {
+
+// The implementation the embedder has to provide for the Presentation API to work.
+class WebPresentationSessionClient {
+public:
+    virtual ~WebPresentationSessionClient() { }
+
+    virtual WebString getId() = 0;
+    virtual WebString getUrl() = 0;
+};
+
+} // namespace blink
+
+#endif // WebPresentationSessionClient_h