[Telemetry] Clear tracing state before browser start

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 d9ea430a2cfe914230057ea1840e7ddd8ccf5e33..454a08794304a9a2e2eb19fc0588dc1acc00940a 100644
--- a/telemetry/telemetry/page/shared_page_state.py
+++ b/telemetry/telemetry/page/shared_page_state.py
@@ -221,6 +221,13 @@ class SharedPageState(story.SharedState):
         logging.warning('System info not supported')
 
   def WillRunStory(self, page):
+    if not self.platform.tracing_controller.is_tracing_running:

[nednguyen, 2016/04/11 18:12:29]

Actually, I think we should move this to browser.Start() method.

+      # For TimelineBasedMeasurement benchmarks, tracing has already started.
+      # For PageTest benchmarks, tracing has not yet started. We need to make
+      # sure no tracing state is left before starting the browser for PageTest
+      # benchmarks.
+      self.platform.tracing_controller.ClearStateIfNeeded()
+
     if self._ShouldDownloadPregeneratedProfileArchive():
       self._DownloadPregeneratedProfileArchive()