[WIP] DevToolsClientBackend with tracing API.

tools/telemetry/telemetry/core/backends/chrome/chrome_browser_backend.py

Index: tools/telemetry/telemetry/core/backends/chrome/chrome_browser_backend.py
diff --git a/tools/telemetry/telemetry/core/backends/chrome/chrome_browser_backend.py b/tools/telemetry/telemetry/core/backends/chrome/chrome_browser_backend.py
index 90d6c32ed5d3616c2b5965d31cb93bfdaae8fc5f..6d992332aa0033a37ac6e8a1679676d07cf04911 100644
--- a/tools/telemetry/telemetry/core/backends/chrome/chrome_browser_backend.py
+++ b/tools/telemetry/telemetry/core/backends/chrome/chrome_browser_backend.py
@@ -23,7 +23,6 @@ from telemetry.core.backends import browser_backend
 from telemetry.core.backends.chrome import extension_backend
 from telemetry.core.backends.chrome import system_info_backend
 from telemetry.core.backends.chrome import tab_list_backend
-from telemetry.core.backends.chrome import tracing_backend
 from telemetry.core.backends.chrome_inspector import devtools_client_backend
 from telemetry.core.backends.chrome_inspector import devtools_http
 from telemetry.timeline import tracing_timeline_data
@@ -46,9 +45,8 @@ class ChromeBrowserBackend(browser_backend.BrowserBackend):
 
     self._platform_backend = platform_backend
     self._supports_tab_control = supports_tab_control
-    self._devtools_client = None
-    self._tracing_backend = None
     self._system_info_backend = None
+    self._devtools_client = None
 
     self._output_profile_path = output_profile_path
     self._extensions_to_load = extensions_to_load
@@ -86,6 +84,14 @@ class ChromeBrowserBackend(browser_backend.BrowserBackend):
       return None
     return extension_backend.ExtensionBackendDict(self)
 
+  @property
+  def devtools_client(self):
+    if not self._devtools_client:
+      assert self._port, 'No DevTools port info available.'
+      self._devtools_client = devtools_client_backend.DevToolsClientBackend(
+          self._port)
+    return self._devtools_client
+
   def GetBrowserStartupArgs(self):
     args = []
     args.extend(self.browser_options.extra_browser_args)
@@ -254,23 +260,6 @@ class ChromeBrowserBackend(browser_backend.BrowserBackend):
   def supports_tracing(self):
     return True
 
-  def StartTracing(self, trace_options, custom_categories=None,
-                   timeout=web_contents.DEFAULT_WEB_CONTENTS_TIMEOUT):
-    """
-    Args:
-        trace_options: An tracing_options.TracingOptions instance.
-        custom_categories: An optional string containing a list of
-                         comma separated categories that will be traced
-                         instead of the default category set.  Example: use
-                         "webkit,cc,disabled-by-default-cc.debug" to trace only
-                         those three event categories.
-    """
-    assert trace_options and trace_options.enable_chrome_trace
-    if self._tracing_backend is None:
-      self._tracing_backend = tracing_backend.TracingBackend(self._port, self)
-    return self._tracing_backend.StartTracing(
-        trace_options, custom_categories, timeout)
-
   def StopTracing(self):
     """ Stops tracing and returns the result as TimelineData object. """
     tab_ids_list = []
@@ -284,7 +273,7 @@ class ChromeBrowserBackend(browser_backend.BrowserBackend):
         if not success:
           raise Exception('Page stomped on console.time')
         tab_ids_list.append(tab.id)
-    trace_events = self._tracing_backend.StopTracing()
+    trace_events = self._devtools_client.StopChromeTracing()
     # Augment tab_ids data to trace events.
     event_data = {'traceEvents' : trace_events, 'tabIds': tab_ids_list}
     return tracing_timeline_data.TracingTimelineData(event_data)
@@ -305,9 +294,7 @@ class ChromeBrowserBackend(browser_backend.BrowserBackend):
     return m.group(1)
 
   def Close(self):
-    if self._tracing_backend:
-      self._tracing_backend.Close()
-      self._tracing_backend = None
+    pass
 
   @property
   def supports_system_info(self):