Make recording basic media metrics optional.

tools/perf/measurements/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 metrics import media
 from telemetry.page import page_measurement
 
+
 class Media(page_measurement.PageMeasurement):
   """The MediaMeasurement class gathers media-related metrics on a page set.
 
   Media metrics recorded are controlled by metrics/media.js.  At the end of the
   test each metric for every media element in the page are reported.
   """
   def __init__(self):
     super(Media, self).__init__('media_metrics')
     self._media_metric = None
 
   def results_are_the_same_on_every_page(self):
     """Results can vary from page to page based on media events taking place."""
     return False
 
   def DidNavigateToPage(self, page, tab):
     """Override to do operations right after the page is navigated."""
     self._media_metric = media.MediaMetric(tab)
     self._media_metric.Start(page, tab)
 
   def MeasurePage(self, page, tab, results):
     """Measure the page's performance."""
     self._media_metric.Stop(page, tab)
     self._media_metric.AddResults(tab, results)
+