[Telemetry] Re-add API to tear down state after each story run

telemetry/telemetry/page/shared_page_state.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.
 
 import logging
 import os
 import shutil
 import sys
 import tempfile
 import zipfile
@@ skipping 211 matching lines @@
 
   def WillRunStory(self, page):
     if self._ShouldDownloadPregeneratedProfileArchive():
       self._DownloadPregeneratedProfileArchive()
 
       if self._ShouldMigrateProfile():
         self._MigratePregeneratedProfile()
 
     page_set = page.page_set
     self._current_page = page
-    if self._test.RestartBrowserBeforeEachPage() or page.startup_url:
+    if self._browser and (self._test.RestartBrowserBeforeEachPage()
+                          or page.startup_url):
+      assert not self.platform.tracing_controller.is_tracing_running, (
+          'Should not restart browser when tracing is already running. For '
+          'TimelineBasedMeasurement (TBM) benchmarks, you should not use '
+          'startup_url. Use benchmark.ShouldTearDownStateAfterEachStoryRun '
+          'instead.')
       self._StopBrowser()
     started_browser = not self.browser
 
     archive_path = page_set.WprFilePathForStory(page)
     # TODO(nednguyen, perezju): Ideally we should just let the network
     # controller raise an exception when the archive_path is not found.
     if archive_path is not None and not os.path.isfile(archive_path):
       logging.warning('WPR archive missing: %s', archive_path)
       archive_path = None
     self.platform.network_controller.StartReplay(
@@ skipping 267 matching lines @@
 class SharedDesktopPageState(SharedPageState):
   _device_type = 'desktop'
 
 
 class SharedTabletPageState(SharedPageState):
   _device_type = 'tablet'
 
 
 class Shared10InchTabletPageState(SharedPageState):
   _device_type = 'tablet_10_inch'