[Telemetry] Shortcut 30s energy measurement if not supported.

This reduces telemetry_perf_unittests by 30s.

BUG=388256

Review URL: https://codereview.chromium.org/474493002

Cr-Commit-Position: refs/heads/master@{#289609}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@289609 0039d316-1c4b-4281-b951-d872f2087c98

tools/perf/measurements/tab_switching.py

@@ -62,10 +62,11 @@ class TabSwitching(page_test.PageTest):
     # Measure power usage of tabs after quiescence.
     util.WaitFor(tab.HasReachedQuiescence, 60)
 
-    self._power_metric.Start(page, tab)
-    time.sleep(TabSwitching.SAMPLE_TIME)
-    self._power_metric.Stop(page, tab)
-    self._power_metric.AddResults(tab, results,)
+    if tab.browser.platform.CanMonitorPower():
+      self._power_metric.Start(page, tab)
+      time.sleep(TabSwitching.SAMPLE_TIME)
+      self._power_metric.Stop(page, tab)
+      self._power_metric.AddResults(tab, results,)
 
     histogram_name = 'MPArch.RWH_TabSwitchPaintDuration'
     histogram_type = histogram_util.BROWSER_HISTOGRAM