[Telemetry] Platform-level tracing

tools/telemetry/telemetry/internal/backends/chrome_inspector/tracing_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 json
 import socket
 import sys
 import time
 
 from telemetry import decorators
@@ skipping 79 matching lines @@
     self._data += result['data']
     if not result.get('eof', False):
       self._ReadChunkFromStream()
       return
     req = {'method': 'IO.close', 'params': {'handle': self._handle}}
     self._inspector_websocket.SendAndIgnoreResponse(req)
     self._callback(self._data)
 
 
 class TracingBackend(object):
-  def __init__(self, devtools_port):
+  def __init__(self, devtools_port, is_tracing_running=False):
     self._inspector_websocket = inspector_websocket.InspectorWebsocket()
     self._inspector_websocket.RegisterDomain(
         'Tracing', self._NotificationHandler)
 
     self._inspector_websocket.Connect(
         'ws://127.0.0.1:%i/devtools/browser' % devtools_port)
     self._trace_events = []
-    self._is_tracing_running = False
+    self._is_tracing_running = is_tracing_running
     self._has_received_all_tracing_data = False
 
   @property
   def is_tracing_running(self):
     return self._is_tracing_running
 
   def StartTracing(self, trace_options, custom_categories=None, timeout=10):
     """When first called, starts tracing, and returns True.
 
     If called during tracing, tracing is unchanged, and it returns False.
@@ skipping 128 matching lines @@
     self._has_received_all_tracing_data = True
 
   def Close(self):
     self._inspector_websocket.Disconnect()
 
   @decorators.Cache
   def IsTracingSupported(self):
     req = {'method': 'Tracing.hasCompleted'}
     res = self._inspector_websocket.SyncRequest(req)
     return not res.get('response')