Implement enumerateDevices() according to spec.

Source/modules/mediastream/MediaDevices.idl

Index: Source/modules/mediastream/MediaDevices.idl
diff --git a/Source/modules/mediastream/MediaDevices.idl b/Source/modules/mediastream/MediaDevices.idl
new file mode 100644
index 0000000000000000000000000000000000000000..e5f2f6b55ccd8192feb72732abcfee3574937b55
--- /dev/null
+++ b/Source/modules/mediastream/MediaDevices.idl
@@ -0,0 +1,13 @@
+// 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.
+
+// http://w3c.github.io/mediacapture-main/#mediadevices
+
+[
+    GarbageCollected,
+    RuntimeEnabled=EnumerateDevices
+]
+interface MediaDevices : EventTarget {
+    [RuntimeEnabled=EnumerateDevices, TypeChecking=Interface, CallWith=ScriptState] Promise<sequence<MediaDeviceInfo>> enumerateDevices();
+};