[Telemetry] fix run_benchmark crash on CrOS

telemetry/telemetry/internal/backends/chrome/chrome_browser_backend.py

 # Copyright 2013 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 pprint
 import shlex
 import sys
 
 from telemetry.core import exceptions
@@ skipping 128 matching lines @@
         self._port,
         self._browser_target, self)
 
   def _WaitForBrowserToComeUp(self, remote_devtools_port=None):
     """ Wait for browser to come up.
 
     Args:
       remote_devtools_port: The remote devtools port, if
           any. Otherwise assumed to be the same as self._port.
     """
+    if self._devtools_client:
+      # In case we are launching a second browser instance (as is done by
+      # the CrOS backend), ensure that the old devtools_client is closed,
+      # otherwise re-creating it will fail.
+      self._devtools_client.Close()
+      self._devtools_client = None
     try:
       timeout = self.browser_options.browser_startup_timeout
       py_utils.WaitFor(self.HasBrowserFinishedLaunching, timeout=timeout)
     except (py_utils.TimeoutException, exceptions.ProcessGoneException) as e:
       if not self.IsBrowserRunning():
         raise exceptions.BrowserGoneException(self.browser, e)
       raise exceptions.BrowserConnectionGoneException(self.browser, e)
     self._devtools_client = devtools_client_backend.DevToolsClientBackend(
         self._port, self._browser_target,
         remote_devtools_port or self._port, self)
@@ skipping 139 matching lines @@
   def supports_cpu_metrics(self):
     return True
 
   @property
   def supports_memory_metrics(self):
     return True
 
   @property
   def supports_power_metrics(self):
     return True