Add MSE perf Telemetry tests.

tools/perf/benchmarks/media.py

 # Copyright 2013 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.
 from measurements import media
 from telemetry import test
 
-
 class Media(test.Test):
   """Obtains media metrics for key user scenarios."""
   test = media.Media
   page_set = 'page_sets/tough_video_cases.json'
+
+class MediaSourceExtensions(test.Test):
+  """Obtains media metrics for key media source extensions functions."""
+  test = media.Media
+  page_set = 'page_sets/mse_cases.json'
+
+  def CustomizeBrowserOptions(self, options):
+    # Needed to allow XHR requests to return stream objects.
+    options.AppendExtraBrowserArgs(
+        '--enable-experimental-web-platform-features')