[Telemetry] fix run_benchmark crash on CrOS

telemetry/telemetry/internal/backends/chrome/cros_browser_backend.py

Index: telemetry/telemetry/internal/backends/chrome/cros_browser_backend.py
diff --git a/telemetry/telemetry/internal/backends/chrome/cros_browser_backend.py b/telemetry/telemetry/internal/backends/chrome/cros_browser_backend.py
index 397b04aa8141ff58963dac77fc586bc8f95825da..9d99ce1302e9b05c4606b7415e143b024eab0cb2 100644
--- a/telemetry/telemetry/internal/backends/chrome/cros_browser_backend.py
+++ b/telemetry/telemetry/internal/backends/chrome/cros_browser_backend.py
@@ -167,7 +167,9 @@ class CrOSBrowserBackend(chrome_browser_backend.ChromeBrowserBackend):
 
     # Wait for new chrome and oobe.
     py_utils.WaitFor(lambda: pid != self.pid, 15)
-    self._WaitForBrowserToComeUp()
+    # Disable tracing for the login-screen instance of Chrome, otherwise
+    # tracing on the instance of Chrome running the benchmark will fail.
+    self._WaitForBrowserToComeUp(enable_tracing=False)
     py_utils.WaitFor(lambda: self.oobe_exists, 30)
 
     if self.browser_options.auto_login:
@@ -192,7 +194,7 @@ class CrOSBrowserBackend(chrome_browser_backend.ChromeBrowserBackend):
             not self.browser_options.disable_gaia_services)
 
       try:
-        self._WaitForLogin()
+        self._WaitForLogin(enable_tracing=True)
       except py_utils.TimeoutException:
         self._RaiseOnLoginFailure('Timed out going through login screen. '
                                   + self._GetLoginStatus())
@@ -290,13 +292,13 @@ class CrOSBrowserBackend(chrome_browser_backend.ChromeBrowserBackend):
     responsive to devtools requests, and the oobe has been dismissed."""
     return not self._GetLoginStatus()
 
-  def _WaitForLogin(self):
+  def _WaitForLogin(self, enable_tracing):
     # Wait for cryptohome to mount.
     py_utils.WaitFor(self._IsLoggedIn, 900)
 
     # For incognito mode, the session manager actually relaunches chrome with
     # new arguments, so we have to wait for the browser to come up.
-    self._WaitForBrowserToComeUp()
+    self._WaitForBrowserToComeUp(enable_tracing)

[achuithb, 2017/08/17 15:29:26]

I think we may be able to get rid of this call, but maybe try that in a
follow-up CL.

 
     # Wait for extensions to load.
     if self._supports_extensions: