[Telemetry] Measure power in tab_switching measurement

tools/telemetry/telemetry/core/web_contents.py

Index: tools/telemetry/telemetry/core/web_contents.py
diff --git a/tools/telemetry/telemetry/core/web_contents.py b/tools/telemetry/telemetry/core/web_contents.py
index dd04e3ce0a9fee4c3be53b8b9fbbe959915238fc..e619ae3222638a27c3494d4d00b735920a5bc2a2 100644
--- a/tools/telemetry/telemetry/core/web_contents.py
+++ b/tools/telemetry/telemetry/core/web_contents.py
@@ -2,6 +2,8 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import os
+
 from telemetry.core import util
 
 DEFAULT_WEB_CONTENTS_TIMEOUT = 90
@@ -13,6 +15,12 @@ class WebContents(object):
   def __init__(self, inspector_backend):
     self._inspector_backend = inspector_backend
 
+    self._has_reached_quiescence = False
+    self._quiescence_js_loaded = False
+    with open(os.path.join(os.path.dirname(__file__),
+        'time_since_last_response.js')) as f:
+      self._quiescence_js = f.read()
+
   def Close(self):
     """Closes this page.
 
@@ -46,6 +54,28 @@ class WebContents(object):
         return False
     util.WaitFor(IsTrue, timeout)
 
+  def HasReachedQuiescence(self):
+    """Determine whether the page has reached quiescence after loading.
+
+    Returns:
+    True if 2 seconds have passed since last resource received, false
+    otherwise."""
+    # The script that provides the function window.timeSinceLastResponseMs()
+    # needs to be loaded for this function, but it must be loaded AFTER
+    # PageMeasurement's WillNavigateToPage function is called, otherwise 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._quiescence_js_loaded:
+      self.ExecuteJavaScript(self._quiescence_js)
+      self._quiescence_js_loaded = True
+
+    if (self._has_reached_quiescence):
+      return True
+    time_since_last_response_ms = (
+        self.EvaluateJavaScript("window.timeSinceLastResponseAfterLoadMs()"))
+    self._has_reached_quiescence = time_since_last_response_ms > 2000
+    return self._has_reached_quiescence
+
   def ExecuteJavaScript(self, expr, timeout=DEFAULT_WEB_CONTENTS_TIMEOUT):
     """Executes expr in JavaScript. Does not return the result.