Introduce TracingController and TracingControllerBackend skeleton

tools/telemetry/telemetry/core/platform/tracing_controller.py

Index: tools/telemetry/telemetry/core/platform/tracing_controller.py
diff --git a/tools/telemetry/telemetry/core/platform/tracing_controller.py b/tools/telemetry/telemetry/core/platform/tracing_controller.py
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index 0000000000000000000000000000000000000000..b6f29359c610bc3e7ad12b50483d4eb089c6932a
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+++ b/tools/telemetry/telemetry/core/platform/tracing_controller.py
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+# 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 TracingController(object):
+  def __init__(self, tracing_controller_backend):
+    """Provides control of the tracing systems supported by telemetry."""
+    self._tracing_controller_backend = tracing_controller_backend
+
+  def Start(self, trace_options, category_filter, timeout=10):
+    """Starts tracing.
+
+    trace_options specifies which tracing systems to activate. Category filter
+    allows fine-tuning of the data that are collected by the selected tracing
+    systems.
+
+    Some tracers are process-specific, e.g. chrome tracing, but are not
+    guaranteed to be supported. In order to support tracing of these kinds of
+    tracers, Start will succeed *always*, even if the tracing systems you have
+    requested are not supported.
+
+    If you absolutely require a particular tracer to exist, then check
+    for its support after you have started the process in question. Or, have
+    your code fail gracefully when the data you require is not present in the
+    resulting trace.
+    """
+    self._tracing_controller_backend.Start(
+        trace_options, category_filter, timeout)
+
+  def Stop(self):
+    """Stops tracing and returns a TraceValue."""
+    return self._tracing_controller_backend.Stop()
+
+  @property
+  def is_tracing_running(self):
+    return self._tracing_controller_backend.is_tracing_running
+
+  def IsChromeTracingSupported(self, browser):
+    """Returns whether chrome tracing is supported on the given browser."""
+    return self._tracing_controller_backend.IsChromeTracingSupported(browser)