[Telemetry] Measure power in tab_switching measurement

tools/perf/metrics/speedindex.py

Index: tools/perf/metrics/speedindex.py
diff --git a/tools/perf/metrics/speedindex.py b/tools/perf/metrics/speedindex.py
index e743f1848b90901b277e5507511052c61aad00ab..6e86d67c3d2aaad31e71690faa0c701d56152bae 100644
--- a/tools/perf/metrics/speedindex.py
+++ b/tools/perf/metrics/speedindex.py
@@ -78,23 +78,7 @@ class SpeedIndexMetric(Metric):
       True if 2 seconds have passed since last resource received, false
       otherwise.
     """
-    if self._is_finished:
-      return True
-
-    # The script that provides the function window.timeSinceLastResponseMs()
-    # needs to be loaded for this function, but it must be loaded AFTER
-    # the Start method is called, because if the Start method is called in
-    # the PageMeasurement's WillNavigateToPage function, then it will
-    # not be available here. The script should only be re-loaded once per page
-    # so that variables in the script get reset only for a new page.
-    if not self._script_is_loaded:
-      tab.ExecuteJavaScript(self._js)
-      self._script_is_loaded = True
-
-    time_since_last_response_ms = tab.EvaluateJavaScript(
-        "window.timeSinceLastResponseAfterLoadMs()")
-    self._is_finished = time_since_last_response_ms > 2000
-    return self._is_finished
+    return tab.HasReachedQuiescence()
 
 
 class SpeedIndexImpl(object):