[Telemetry] fix run_benchmark crash on CrOS

telemetry/telemetry/internal/backends/chrome_inspector/devtools_client_backend.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 re
 import socket
 import sys
 
 from telemetry.core import exceptions
@@ skipping 71 matching lines @@
     return True
 
 
 class DevToolsClientBackend(object):
   """An object that communicates with Chrome's devtools.
 
   This class owns a map of InspectorBackends. It is responsible for creating
   them and destroying them.
   """
   def __init__(self, devtools_port, browser_target, remote_devtools_port,
-               app_backend):
+               app_backend, enable_tracing=True):
     """Creates a new DevToolsClientBackend.
 
     A DevTools agent must exist on the given devtools_port.
 
     Args:
       devtools_port: The port to use to connect to DevTools agent.
       remote_devtools_port: In some cases (e.g., app running on
           Android device, devtools_port is the forwarded port on the
           host platform. We also need to know the remote_devtools_port
           so that we can uniquely identify the DevTools agent.
       app_backend: For the app that contains the DevTools agent.
     """
     self._devtools_port = devtools_port
     self._browser_target = browser_target or '/devtools/browser'
     self._remote_devtools_port = remote_devtools_port
     self._devtools_http = devtools_http.DevToolsHttp(devtools_port)
     self._browser_inspector_websocket = None
     self._tracing_backend = None
     self._memory_backend = None
     self._system_info_backend = None
     self._app_backend = app_backend
     self._devtools_context_map_backend = _DevToolsContextMapBackend(
         self._app_backend, self)
+    self._enable_tracing = enable_tracing
 
     self._tab_ids = None
 
     if not self.supports_tracing:
       return
     chrome_tracing_devtools_manager.RegisterDevToolsClient(
         self, self._app_backend.platform_backend)
 
     # Telemetry has started Chrome tracing if there is trace config, so start
     # tracing on this newly created devtools client if needed.
@@ skipping 11 matching lines @@
     self.StartChromeTracing(trace_config)
 
   @property
   def remote_port(self):
     return self._remote_devtools_port
 
   @property
   def supports_tracing(self):
     if not isinstance(self._app_backend, browser_backend.BrowserBackend):
       return False
+    if not self._enable_tracing:

[nednguyen, 2017/08/17 14:16:24]

This is a misusage of abstraction. The name of this property is
"supports_tracing", whereas what you want is not to enable tracing even if it's
supported.

+      return False
     return self._app_backend.supports_tracing
 
   @property
   def supports_overriding_memory_pressure_notifications(self):
     if not isinstance(self._app_backend, browser_backend.BrowserBackend):
       return False
     return self._app_backend.supports_overriding_memory_pressure_notifications
 
 
   @property
@@ skipping 353 matching lines @@
                         context_id)
           continue
       valid_contexts.append(context)
     self._contexts = valid_contexts
 
   def Clear(self):
     for backend in self._inspector_backends_dict.values():
       backend.Disconnect()
     self._inspector_backends_dict = {}
     self._contexts = None