[Telemetry] Add benchmark that doesn't launch chrome and collects platform data.

tools/perf/page_sets/idle_platform.py

+# Copyright 2017 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.
+import time
+
+from telemetry.page import shared_page_state
+from telemetry import story
+
+
+class _IdleSharedState(shared_page_state.SharedPageState):
+  def __init__(self, test, finder_options, story_set):
+    super(_IdleSharedState, self).__init__(test, finder_options, story_set)
+    self._current_story = None
+
+  def WillRunStory(self, current_story):
+    self._current_story = current_story
+    if not self.platform.tracing_controller.is_tracing_running:
+      # For TimelineBasedMeasurement benchmarks, tracing has already started.
+      # For PageTest benchmarks, tracing has not yet started. We need to make
+      # sure no tracing state is left before starting the browser for PageTest
+      # benchmarks.

[nednguyen, 2017/03/30 22:01:40]

We don't need to worry about supporting this for legacy_page_test benchmark. I
think you can just add assert that tracing is running.

[rnephew (Reviews Here), 2017/04/03 16:25:35]

Done.

+      self.platform.tracing_controller.ClearStateIfNeeded()
+
+  def RunStory(self, _):
+    self._current_story.Run(self)
+
+  def DidRunStory(self, _):
+    self._current_story = None
+
+
+class IdleStory(story.Story):

[nednguyen, 2017/03/30 22:07:56]

make this private: _IdleStory

[rnephew (Reviews Here), 2017/04/03 16:25:35]

Done.

+  def __init__(self, name, duration=30):

[nednguyen, 2017/03/30 22:07:55]

No need to set a default param here if this is only internal.

[rnephew (Reviews Here), 2017/04/03 16:25:35]

Done.

+    super(IdleStory, self).__init__(
+        shared_state_class=_IdleSharedState, name=name)
+    self._duration = duration
+
+  def Run(self, shared_state):
+    time.sleep(self._duration)
+
+
+class IdleStorySet(story.StorySet):
+  def __init__(self):
+    super(IdleStorySet, self).__init__(
+        archive_data_file=None)
+    self.AddStory(IdleStory('Idle Platform'))

[nednguyen, 2017/03/30 22:07:56]

Can you create 3 stories: 
self.AddStory(IdleStory('IdleStory_10s'), 10)
self.AddStory(IdleStory('IdleStory_60s'), 60)
self.AddStory(IdleStory('IdleStory_120s'), 120)

[rnephew (Reviews Here), 2017/04/03 16:25:35]

Done.