Add a Telemetry based cold startup test.

tools/perf/measurements/startup.py

 # Copyright (c) 2013 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 json
 
 from telemetry.page import page_measurement
 
+class Startup(page_measurement.PageMeasurement):
+  """Performs a measurement of Chromium's startup performance.
 
-class StartupWarm(page_measurement.PageMeasurement):
-  """Test how long Chrome takes to load when warm."""
+  This test must be invoked with either --warm or --cold on the command line. A
+  cold start means none of the Chromium files are in the disk cache. A warm
+  start assumes the OS has already cached much of Chromium's content. For warm
+  tests, you should repeat the page set to ensure it's cached.
+  """
+
   HISTOGRAMS_TO_RECORD = {
     'messageloop_start_time' :
         'Startup.BrowserMessageLoopStartTimeFromMainEntry',
     'window_display_time' : 'Startup.BrowserWindowDisplay',
     'open_tabs_time' : 'Startup.BrowserOpenTabs'}
 
   def __init__(self):
-    super(StartupWarm, self).__init__(needs_browser_restart_after_each_run=True,
-                                      discard_first_result=True)
+    super(Startup, self).__init__(needs_browser_restart_after_each_run=True)
+    self._cold = False
+
+  def AddCommandLineOptions(self, parser):
+    parser.add_option('--cold', action='store_true',
+                      help='Clear the OS disk cache before performing the test')
+    parser.add_option('--warm', action='store_true',
+                      help='Start up with everything already cached')
 
   def CustomizeBrowserOptions(self, options):
+    # TODO: Once the bots start running benchmarks, enforce that either --warm
+    # or --cold is explicitly specified.
+    # assert options.warm != options.cold, \
+    #     "You must specify either --warm or --cold"
+    self._cold = options.cold
+
+    if self._cold:
+      options.clear_sytem_cache_for_browser_and_profile_on_start = True
+    else:
+      self.discard_first_result = True
+
     options.AppendExtraBrowserArg('--enable-stats-collection-bindings')
 
     # Old commandline flags used for reference builds.
     options.AppendExtraBrowserArg('--dom-automation')
     options.AppendExtraBrowserArg(
           '--reduce-security-for-dom-automation-tests')
 
   def MeasurePage(self, page, tab, results):
     # TODO(jeremy): Remove references to
     # domAutomationController.getBrowserHistogram when we update the reference
     # builds.
     get_histogram_js = ('(window.statsCollectionController ?'
         'statsCollectionController :'
         'domAutomationController).getBrowserHistogram("%s")')
 
-
     for display_name, histogram_name in self.HISTOGRAMS_TO_RECORD.iteritems():
       result = tab.EvaluateJavaScript(get_histogram_js % histogram_name)
       result = json.loads(result)
       measured_time = 0
 
       if 'sum' in result:
         # For all the histograms logged here, there's a single entry so sum
         # is the exact value for that entry.
         measured_time = result['sum']
       elif 'buckets' in result:
         measured_time = \
             (result['buckets'][0]['high'] + result['buckets'][0]['low']) / 2
 
       results.Add(display_name, 'ms', measured_time)