Remove start & stop logic from timeline_metric.

tools/perf/measurements/timeline_based_measurement.py

 # 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 metrics import timeline as timeline_module
+from measurements import timeline_controller
 from metrics import timeline_interaction_record as tir_module
 from metrics import smoothness
 from telemetry.page import page_measurement
 from telemetry.core.timeline import model as model_module
 
 
 # TimelineBasedMeasurement considers all instrumentation as producing a single
 # timeline. But, depending on the amount of instrumentation that is enabled,
 # overhead increases. The user of the measurement must therefore chose between
 # a few levels of instrumentation.
@@ skipping 84 matching lines @@
         '--overhead-level', type='choice',
         choices=ALL_OVERHEAD_LEVELS,
         default=NO_OVERHEAD_LEVEL,
         help='How much overhead to incur during the measurement.')
 
   def WillNavigateToPage(self, page, tab):
     if not tab.browser.supports_tracing:
       raise Exception('Not supported')
     assert self.options.overhead_level in ALL_OVERHEAD_LEVELS
     if self.options.overhead_level == NO_OVERHEAD_LEVEL:
-      categories = timeline_module.MINIMAL_TRACE_CATEGORIES
+      categories = timeline_controller.MINIMAL_TRACE_CATEGORIES
     elif self.options.overhead_level == \
         MINIMAL_OVERHEAD_LEVEL:
       categories = ''
     else:
       categories = '*,disabled-by-default-cc.debug'
     categories = ','.join([categories] + page.GetSyntheticDelayCategories())
     tab.browser.StartTracing(categories)
 
   def MeasurePage(self, page, tab, results):
     """ Collect all possible metrics and added them to results. """
     trace_result = tab.browser.StopTracing()
     model = model_module.TimelineModel(trace_result)
     renderer_thread = model.GetRendererThreadFromTab(tab)
     meta_metrics = _TimelineBasedMetrics(model, renderer_thread)
     meta_metrics.AddResults(results)