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

telemetry/telemetry/page/shared_page_state.py

Index: telemetry/telemetry/page/shared_page_state.py
diff --git a/telemetry/telemetry/page/shared_page_state.py b/telemetry/telemetry/page/shared_page_state.py
index 3c0e07d094df069ebce239f151dbb60587d8ca50..0f8cbe4e0347b07e014dccc4e583ac8ae15dfca5 100644
--- a/telemetry/telemetry/page/shared_page_state.py
+++ b/telemetry/telemetry/page/shared_page_state.py
@@ -229,7 +229,13 @@ class SharedPageState(story.SharedState):
 
     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