Add navigator.mediaDevices.getUserMedia(), deprecate old method (#342)

The `navigator.getUserMedia()` method is deprecated:
https://developer.mozilla.org/en-US/docs/Web/API/Navigator/getUserMedia

The deprecation seems to have occurred around 28th May 2014:
w3c/mediacapture-main@de3bdaf#diff-ea76d38900f79cfae8f60e5f7cf16dd1R2755

Author: rtyley

src/main/scala/org/scalajs/dom/experimental/webrtc/package.scala

@@ -12,6 +12,19 @@ package object webrtc {
   @js.native
   trait NavigatorMediaStream extends js.Object {
 
+    /**
+     * The Navigator.mediaDevices read-only property returns a MediaDevices object, which provides access to connected
+     * media input devices like cameras and microphones, as well as screen sharing.
+     *
+     * MDN
+     *
+     * @see [[https://developer.mozilla.org/en-US/docs/Web/API/Navigator/mediaDevices]]
+     */
+    def mediaDevices: MediaDevices = js.native
+
+    @deprecated(
+        "Navigator.getUserMedia() is deprecated, use Navigator.mediaDevices.getUserMedia() instead.",
+        "2014-05-28")
     def getUserMedia(constraints: MediaStreamConstraints,
         success: js.Function1[MediaStream, Any],
         error: js.Function1[DOMError, Any]): Unit = js.native