Implement enumerateDevices() according to spec.

Source/modules/mediastream/NavigatorUserMedia.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 NavigatorUserMedia_h
+#define NavigatorUserMedia_h
+
+#include "platform/Supplementable.h"
+#include "platform/heap/Handle.h"
+
+namespace blink {
+
+class Navigator;
+class MediaDevices;
+
+class NavigatorUserMedia final : public GarbageCollected<NavigatorUserMedia>, public HeapSupplement<Navigator> {
+    USING_GARBAGE_COLLECTED_MIXIN(NavigatorUserMedia)
+public:
+    static MediaDevices* mediaDevices(Navigator&);
+    DECLARE_VIRTUAL_TRACE();
+
+private:
+    NavigatorUserMedia();
+    MediaDevices* getMediaDevices();
+    static const char* supplementName();
+    static NavigatorUserMedia& from(Navigator&);
+
+    Member<MediaDevices> m_mediaDevices;
+};
+
+} // namespace blink
+
+#endif