Remove start & stop logic from timeline_metric.

tools/perf/measurements/timeline_based_measurement.py

Index: tools/perf/measurements/timeline_based_measurement.py
diff --git a/tools/perf/measurements/timeline_based_measurement.py b/tools/perf/measurements/timeline_based_measurement.py
index 4c4f67f917103705eb99b3d9737af1054c8f93dd..a4de79d505431e49baa06bd33dc68e290c88670a 100644
--- a/tools/perf/measurements/timeline_based_measurement.py
+++ b/tools/perf/measurements/timeline_based_measurement.py
@@ -2,7 +2,7 @@
 # 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
@@ -107,7 +107,7 @@ class TimelineBasedMeasurement(page_measurement.PageMeasurement):
       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 = ''