[WIP] DevToolsClientBackend with tracing API.

tools/telemetry/telemetry/core/backends/chrome_inspector/devtools_client_backend.py

Index: tools/telemetry/telemetry/core/backends/chrome_inspector/devtools_client_backend.py
diff --git a/tools/telemetry/telemetry/core/backends/chrome_inspector/devtools_client_backend.py b/tools/telemetry/telemetry/core/backends/chrome_inspector/devtools_client_backend.py
index 0489927e5d141a0d3a50b45642c2c9b6f9872ff2..333ed60442ae996df157f5e63159074a0f0e9829 100644
--- a/tools/telemetry/telemetry/core/backends/chrome_inspector/devtools_client_backend.py
+++ b/tools/telemetry/telemetry/core/backends/chrome_inspector/devtools_client_backend.py
@@ -5,7 +5,9 @@
 import re
 
 from telemetry import decorators
+from telemetry.core import exceptions
 from telemetry.core.backends.chrome_inspector import devtools_http
+from telemetry.core.backends.chrome_inspector import tracing_backend
 
 
 class DevToolsClientBackend(object):
@@ -55,3 +57,31 @@ class DevToolsClientBackend(object):
   # equivalent. crbug.com/423954.
   def ListInspectableContexts(self):
     return self._devtools_http.RequestJson('')
+
+  @property
+  def support_tracing(self):
+    # TODO(chrishenry): Is there a case where DevTools server is
+    # available but tracing is not supported? If not, we should delete
+    # this method.
+    return True
+
+  def StartChromeTracing(
+      self, trace_options, custom_categories=None, timeout=10):
+    """
+    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.
+    """
+    if not self._tracing_backend:
+      self._tracing_backend = tracing_backend.TracingBackend(
+          self._devtools_port, self)
+    return self._tracing_backend.StartTracing(
+        trace_options, custom_categories, timeout)
+
+  def StopChromeTracing(self, timeout=30):
+    assert self._tracing_backend
+    return self._tracing_backend.StopTracing(timeout)