[Android] Don't wait for background tabs

tools/perf/measurements/session_restore.py

Index: tools/perf/measurements/session_restore.py
diff --git a/tools/perf/measurements/session_restore.py b/tools/perf/measurements/session_restore.py
index 5ad03a1065fe7d222aabeb4d68b23a4a722c314e..b889f232020a1fe7141d2a6da5f75a08a9a8dc25 100644
--- a/tools/perf/measurements/session_restore.py
+++ b/tools/perf/measurements/session_restore.py
@@ -50,11 +50,14 @@ class SessionRestore(startup.Startup):
     self._cpu_metric.Start(None, None)
 
   def MeasurePage(self, page, tab, results):
-    # Wait for all tabs to finish loading.
-    for i in xrange(len(tab.browser.tabs)):
-      t = tab.browser.tabs[i]
-      t.WaitForDocumentReadyStateToBeComplete()
-
+    # Wait for foreground tab to finish loading. We can't wait for all tabs to
+    # finish loading because on Android the data of the background tabs is
+    # loaded on demand, when the user switches to them, rather than during
+    # startup.
+    # We are assuming that the foreground tab is tab[0]. This is true in current
+    # code, but is not guaranteed by the APIs.
+    t = tab.browser.tabs[0]

[jeremy, 2014/02/13 13:35:26]

Looks like we have no way of checking that this is in fact the frontmost tab
which kind of scares me :/

[aberent, 2014/02/13 15:59:30]

Fixed, I think (just testing) although it only works through luck on Android
(due to a bug). See the next patch.

+    t.WaitForDocumentReadyStateToBeComplete()
     # Record CPU usage from browser start to when all pages have loaded.
     self._cpu_metric.Stop(None, None)
     self._cpu_metric.AddResults(tab, results, 'cpu_utilization')