Fix CSV header output for TimelineBasedMeasurement.

This is done by setting results_are_the_same_on_every_page property on
TimelineBasedMeasurement to False.

BUG=393049

Review URL: https://codereview.chromium.org/387783002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@282506 0039d316-1c4b-4281-b951-d872f2087c98

tools/telemetry/telemetry/web_perf/timeline_based_measurement.py

@@ -159,3 +159,7 @@ class TimelineBasedMeasurement(page_measurement.PageMeasurement):
   def CleanUpAfterPage(self, page, tab):
     if tab.browser.is_tracing_running:
       tab.browser.StopTracing()
+
+  @property
+  def results_are_the_same_on_every_page(self):
+    return False