Implement enumerateDevices() according to spec.

Source/modules/mediastream/NavigatorUserMedia.idl

Index: Source/modules/mediastream/NavigatorUserMedia.idl
diff --git a/Source/modules/plugins/NavigatorPlugins.idl b/Source/modules/mediastream/NavigatorUserMedia.idl
similarity index 50%
copy from Source/modules/plugins/NavigatorPlugins.idl
copy to Source/modules/mediastream/NavigatorUserMedia.idl
index 97b209aa2d3cc1f9401f4bd9d80521e4bd061cf0..8c38ca130846abe2fe71a5e8e5c6e4ac46ddbe21 100644
--- a/Source/modules/plugins/NavigatorPlugins.idl
+++ b/Source/modules/mediastream/NavigatorUserMedia.idl
@@ -2,9 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// https://html.spec.whatwg.org/multipage/webappapis.html#navigatorplugins
+// http://w3c.github.io/mediacapture-main/#navigatorusermedia
+
 partial interface Navigator {
-    readonly attribute PluginArray plugins;
-    readonly attribute MimeTypeArray mimeTypes;
-    boolean javaEnabled();
+    [RuntimeEnabled=EnumerateDevices] readonly attribute MediaDevices mediaDevices;
 };
+