[Telemetry] Add tracing_agent API.

tools/telemetry/telemetry/core/platform/tracing_agent/display_tracing_agent.py

Index: tools/telemetry/telemetry/core/platform/tracing_agent/display_tracing_agent.py
diff --git a/tools/telemetry/telemetry/core/platform/tracing_agent/display_tracing_agent.py b/tools/telemetry/telemetry/core/platform/tracing_agent/display_tracing_agent.py
new file mode 100644
index 0000000000000000000000000000000000000000..3ae0ebb6621c24fda7376138514e1181bae5f843
--- /dev/null
+++ b/tools/telemetry/telemetry/core/platform/tracing_agent/display_tracing_agent.py
@@ -0,0 +1,26 @@
+# Copyright 2014 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.
+
+from telemetry.core.platform import tracing_agent
+from telemetry.timeline import trace_data
+
+
+class DisplayTracingAgent(tracing_agent.TracingAgent):
+  def __init__(self, platform_backend):
+    super(DisplayTracingAgent, self).__init__(platform_backend)
+
+  @classmethod
+  def IsSupported(cls, platform_backend):
+    return platform_backend.IsDisplayTracingSupported()
+
+  def Start(self, trace_options, category_filter, _timeout):
+    if trace_options.enable_platform_display_trace:
+      self._platform_backend.StartDisplayTracing()
+      return True
+
+  def Stop(self, trace_data_builder):
+    assert self._is_tracing_running
+    surface_flinger_trace_data = self._platform_backend.StopDisplayTracing()
+    trace_data_builder.AddEventsTo(
+          trace_data.SURFACE_FLINGER_PART, surface_flinger_trace_data)