[Telemetry] Add tracing_agent API.

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

Index: tools/telemetry/telemetry/core/platform/tracing_agent/__init__.py
diff --git a/tools/telemetry/telemetry/core/platform/tracing_agent/__init__.py b/tools/telemetry/telemetry/core/platform/tracing_agent/__init__.py
new file mode 100644
index 0000000000000000000000000000000000000000..cad469905682bf47c74644e728f5ffef18312fa4
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+++ b/tools/telemetry/telemetry/core/platform/tracing_agent/__init__.py
@@ -0,0 +1,51 @@
+# 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.
+
+
+class TracingAgent(object):
+  """A tracing agent provided by the platform.
+
+  A tracing agent can gather data with Start() until Stop().
+  Before constructing an TracingAgent, check whether it's supported on the
+  platform with IsSupported method first.
+
+  NOTE: All subclasses of TracingAgent must not change the constructor's
+  parameters so the agents can be dynamically constructed in
+  tracing_controller_backend.
+
+  """
+
+  def __init__(self, platform_backend):
+    self._platform_backend = platform_backend
+
+  @classmethod
+  def IsSupported(cls, _platform_backend):
+    return False
+
+  def Start(self, trace_options, category_filter, timeout):
+    """ Override to add tracing agent's custom logic to start tracing.
+
+    Depending on trace_options and category_filter, the tracing agent may choose
+    to start or not start tracing.
+
+    Args:
+      trace_options: an instance of tracing_options.TracingOptions that
+        control which core tracing systems should be enabled.
+      category_filter: an instance of
+        tracing_category_filter.TracingCategoryFilter
+      timeout: number of seconds that this tracing agent should try to start
+        tracing until time out.
+
+    Returns:
+      True if tracing agent started succesfully.
+    """
+    raise NotImplementedError
+
+  def Stop(self, trace_data_builder):
+    """ Override to add tracing agent's custom logic to stop tracing.
+
+    Caller must check whether tracing is active with IsActive() first before
+    invoking this method.
+    """
+    raise NotImplementedError