[Telemetry] fix run_benchmark crash on CrOS

telemetry/telemetry/internal/backends/chrome_inspector/devtools_client_backend.py

Index: telemetry/telemetry/internal/backends/chrome_inspector/devtools_client_backend.py
diff --git a/telemetry/telemetry/internal/backends/chrome_inspector/devtools_client_backend.py b/telemetry/telemetry/internal/backends/chrome_inspector/devtools_client_backend.py
index 1c8aa90e869c1235fe98a3a9056b485f7030d3a2..ba46e3ea0b75f53ebefad26b25108bcb63951173 100644
--- a/telemetry/telemetry/internal/backends/chrome_inspector/devtools_client_backend.py
+++ b/telemetry/telemetry/internal/backends/chrome_inspector/devtools_client_backend.py
@@ -89,7 +89,7 @@ class DevToolsClientBackend(object):
   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.
@@ -113,6 +113,7 @@ class DevToolsClientBackend(object):
     self._app_backend = app_backend
     self._devtools_context_map_backend = _DevToolsContextMapBackend(
         self._app_backend, self)
+    self._enable_tracing = enable_tracing
 
     self._tab_ids = None
 
@@ -144,6 +145,8 @@ class DevToolsClientBackend(object):
   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