[tools/perf] Tear down state between webview_startup stories

tools/perf/benchmarks/system_health.py

 # Copyright 2016 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.
 
 import re
 
 from core import perf_benchmark
 from telemetry import benchmark
 from telemetry.timeline import tracing_category_filter
 from telemetry.web_perf import timeline_based_measurement
@@ skipping 111 matching lines @@
 
   def CreateTimelineBasedMeasurementOptions(self):
     options = timeline_based_measurement.Options()
     options.SetTimelineBasedMetric('webviewStartupMetric')
     options.config.enable_atrace_trace = True
     options.config.enable_chrome_trace = False
     options.config.atrace_config.app_name = 'org.chromium.webview_shell'
     return options
 
   @classmethod
+  def ShouldTearDownStateAfterEachStoryRun(cls):
+    return True
+
+  @classmethod
   def Name(cls):
-    return 'system_health.webview_startup'
+    return 'system_health.webview_startup'