Implement navigator.mediaDevices.getUserMedia()

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
index 8f6d0dfecd49a708603c0bf7772104b607d077cf..38c43defda5ed47e402f0289e6a4b36de651d2fc 100644
--- a/Source/modules/mediastream/MediaDevices.idl
+++ b/Source/modules/mediastream/MediaDevices.idl
@@ -2,7 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// The spec for MediaDevices is in two parts:
 // http://w3c.github.io/mediacapture-main/#mediadevices
+// http://w3c.github.io/mediacapture-main/#mediadevices-interface-extensions
 
 [
     GarbageCollected,
@@ -10,4 +12,5 @@
 ]
 interface MediaDevices {
     [RuntimeEnabled=EnumerateDevices, TypeChecking=Interface, CallWith=ScriptState] Promise<sequence<MediaDeviceInfo>> enumerateDevices();
+    [CallWith=ScriptState, RaisesException] Promise<MediaStream> getUserMedia(Dictionary options);
 };