Add cleanup for measurements that use tracing

tools/perf/measurements/timeline_based_measurement.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 metrics import timeline as timeline_module
 from metrics import timeline_interaction_record as tir_module
 from metrics import smoothness
 from telemetry.page import page_measurement
 from telemetry.core.timeline import model as model_module
 
@@ skipping 105 matching lines @@
     categories = ','.join([categories] + page.GetSyntheticDelayCategories())
     tab.browser.StartTracing(categories)
 
   def MeasurePage(self, page, tab, results):
     """ Collect all possible metrics and added them to results. """
     trace_result = tab.browser.StopTracing()
     model = model_module.TimelineModel(trace_result)
     renderer_thread = model.GetRendererThreadFromTab(tab)
     meta_metrics = _TimelineBasedMetrics(model, renderer_thread)
     meta_metrics.AddResults(results)
+
+  def CleanUpAfterPage(self, page, tab):
+    if tab.browser.is_tracing_running:
+      tab.browser.StopTracing()