[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

@@ -62,6 +62,7 @@ class TabSwitching(page_test.PageTest):
     # Measure power usage of tabs after quiescence.
     util.WaitFor(tab.HasReachedQuiescence, 60)
 
+    if tab.browser.platform.CanMonitorPower():
       self._power_metric.Start(page, tab)
       time.sleep(TabSwitching.SAMPLE_TIME)
       self._power_metric.Stop(page, tab)