Implement enumerateDevices() according to spec.

Source/modules/mediastream/MediaDevices.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 MediaDevices_h
+#define MediaDevices_h
+
+#include "bindings/core/v8/ScriptPromise.h"
+#include "bindings/core/v8/ScriptWrappable.h"
+
+namespace blink {
+
+class ScriptState;
+
+class MediaDevices final : public GarbageCollected<MediaDevices>, public ScriptWrappable {
+    DEFINE_WRAPPERTYPEINFO();
+public:
+    static MediaDevices* create()
+    {
+        return new MediaDevices();
+    }
+
+    ScriptPromise enumerateDevices(ScriptState*);
+
+    DEFINE_INLINE_TRACE() { }
+
+private:
+    MediaDevices() { }
+};
+
+} // namespace blink
+
+#endif