For page cyclers, move power metrics start to WillNavigate.

Page cycler are considering page loads and not interaction with the
page, so the power metric must take the page load into account.

R=tonyg@chromium.org
BUG=361491

Review URL: https://codereview.chromium.org/281553002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@270075 0039d316-1c4b-4281-b951-d872f2087c98

tools/perf/measurements/page_cycler.py

@@ -82,10 +82,10 @@ class PageCycler(page_measurement.PageMeasurement):
     if self._report_speed_index:
       self._speedindex_metric.Start(page, tab)
     self._cpu_metric.Start(page, tab)
+    self._power_metric.Start(page, tab)
 
   def DidNavigateToPage(self, page, tab):
     self._memory_metric.Start(page, tab)
-    self._power_metric.Start(page, tab)
     if self._record_v8_object_stats:
       self._v8_object_stats_metric.Start(page, tab)