Telemetry / Devtools TraceHandler: exposes tracing via dev tools.

tools/telemetry/telemetry/browser_backend.py

Index: tools/telemetry/telemetry/browser_backend.py
diff --git a/tools/telemetry/telemetry/browser_backend.py b/tools/telemetry/telemetry/browser_backend.py
index c4a0b1eb02355fe578b769ab625e579aee45b645..40f8917c99ece88fc6f8754d8041824342f7538c 100644
--- a/tools/telemetry/telemetry/browser_backend.py
+++ b/tools/telemetry/telemetry/browser_backend.py
@@ -173,7 +173,13 @@ class BrowserBackend(object):
     self._webkit_base_revision = 106313
 
   def Request(self, path, timeout=None):
-    url = 'http://localhost:%i/json' % self._port
+    json_path = 'json'
+    if path:
+      json_path += '/' + path
+    return self.RequestDevToolsPath(json_path, timeout)
+
+  def RequestDevToolsPath(self, path, timeout=None):
+    url = 'http://localhost:%i' % self._port
     if path:
       url += '/' + path
     req = urllib2.urlopen(url, timeout=timeout)
@@ -183,6 +189,22 @@ class BrowserBackend(object):
   def supports_tab_control(self):
     return self._chrome_branch_number >= 1303
 
+  @property
+  def supports_tracing(self):
+    return True
+
+  def StartTracing(self):
+    return self.RequestDevToolsPath('trace/start') == 'ok'
+
+  def StopTracing(self):
+    return self.RequestDevToolsPath('trace/stop') == 'ok'
+
+  def GetTrace(self):
+    def IsTracingRunning(self):
+      return self.RequestDevToolsPath('trace/has_completed') == 'pending'
+    util.WaitFor(lambda: not IsTracingRunning(self), 10)
+    return '{"traceEvents":[' + self.RequestDevToolsPath('trace/get') + ']}'
+
   def CreateForwarder(self, host_port):
     raise NotImplementedError()