[Telemetry] Fix race in session restore measurement.

The previous code made it possible to try to access tabs prior to the session
restore completing. This should make the benchmark much more deterministic.

BUG=375979

Review URL: https://codereview.chromium.org/313923003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@275509 0039d316-1c4b-4281-b951-d872f2087c98

tools/perf/measurements/session_restore.py

 # Copyright 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
+
 import collections
 
 from measurements import startup
 from metrics import cpu
+from metrics import histogram_util
 from metrics import startup_metric
 from telemetry.core import util
 
+
 class SessionRestore(startup.Startup):
   """Performs a measurement of Chromium's Session restore performance.
 
@@ -23,14 +26,20 @@ class SessionRestore(startup.Startup):
 
   def CustomizeBrowserOptions(self, options):
     super(SessionRestore, self).CustomizeBrowserOptions(options)
+    histogram_util.CustomizeBrowserOptions(options)
     options.AppendExtraBrowserArgs([
         '--restore-last-session'
     ])
 
   def TabForPage(self, page, browser):
     # Detect that the session restore has completed.
-    util.WaitFor(lambda: len(browser.tabs), 30)
-    return browser.tabs[0]
+    util.WaitFor(lambda: browser.tabs and
+                 histogram_util.GetHistogramCount(
+                     histogram_util.BROWSER_HISTOGRAM,
+                     'SessionRestore.AllTabsLoaded',
+                     browser.foreground_tab),
+                 30)
+    return browser.foreground_tab
 
   def CanRunForPage(self, page):
     # No matter how many pages in the pageset, just perform one test iteration.
@@ -59,7 +68,7 @@ class SessionRestore(startup.Startup):
     self._cpu_metric.Start(None, None)
 
   def MeasurePage(self, page, tab, results):
-    tab.browser.foreground_tab.WaitForDocumentReadyStateToBeComplete()
+    tab.WaitForDocumentReadyStateToBeComplete()
 
     # Record CPU usage from browser start to when the foreground page is loaded.
     self._cpu_metric.Stop(None, None)