Media Capabilities: update Blink implementation and WPT to match latest spec changes.

third_party/WebKit/Source/modules/mediacapabilities/MediaConfiguration.idl

 // Copyright 2017 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.
 
 // https://wicg.github.io/media-capabilities/#dictdef-mediaconfiguration
 
 enum MediaConfigurationType {
     "file",
-    "MediaSource",
+    "media-source",
 };
 
 dictionary MediaConfiguration {
     required MediaConfigurationType type;
 
     VideoConfiguration video;
     AudioConfiguration audio;
 };