Make sure that tracing is always cleaned up for v8_gc_times page_test

tools/perf/measurements/v8_gc_times.py

 # Copyright 2015 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 telemetry.core.platform import tracing_category_filter
 from telemetry.core.platform import tracing_options
 from telemetry.page import page_test
 from telemetry.timeline.model import TimelineModel
 from telemetry.util import statistics
 from telemetry.value import scalar
@@ skipping 24 matching lines @@
 
     tab.browser.platform.tracing_controller.Start(
         options, category_filter, self._TIME_OUT_IN_SECONDS)
 
   def ValidateAndMeasurePage(self, page, tab, results):
     trace_data = tab.browser.platform.tracing_controller.Stop()
     timeline_model = TimelineModel(trace_data)
     renderer_process = timeline_model.GetRendererProcessFromTabId(tab.id)
     self._AddV8MetricsToResults(renderer_process, results)
 
+  def CleanUpAfterPage(self, page, tab):
+    if tab.browser.platform.tracing_controller.is_tracing_running:
+      tab.browser.platform.tracing_controller.Stop()
+
   def _AddV8MetricsToResults(self, process, results):
     if process is None:
       return
 
     for thread in process.threads.values():
       if thread.name != self._RENDERER_MAIN_THREAD:
         continue
 
       self._AddV8EventStatsToResults(thread, results)
       self._AddCpuTimeStatsToResults(thread, results)
@@ skipping 128 matching lines @@
     self.idle_task_overrun_duration = 0.0
 
   @property
   def thread_duration_outside_idle(self):
     return self.thread_duration - self.thread_duration_inside_idle
 
   @property
   def percentage_thread_duration_during_idle(self):
     return statistics.DivideIfPossibleOrZero(
         100 * self.thread_duration_inside_idle, self.thread_duration)