[Telemetry] Change story disabling in webrtc pageset to StoryExpectations.

tools/perf/benchmarks/webrtc.py

 # Copyright 2014 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 core import perf_benchmark
 
 import page_sets
 from telemetry import benchmark
 from telemetry.timeline import chrome_trace_category_filter
 from telemetry.web_perf import timeline_based_measurement
@@ skipping 24 matching lines @@
     options = timeline_based_measurement.Options(category_filter)
     options.SetTimelineBasedMetrics([
         'cpuTimeMetric',
         'webrtcRenderingMetric',
     ])
     return options
 
   def SetExtraBrowserOptions(self, options):
     options.AppendExtraBrowserArgs('--use-fake-device-for-media-stream')
     options.AppendExtraBrowserArgs('--use-fake-ui-for-media-stream')
+
+  def GetExpectations(self):
+    return page_sets.WebrtcExpectations()