Write "logout-started" event on next boot.

chrome/common/pref_names.cc

Index: chrome/common/pref_names.cc
diff --git a/chrome/common/pref_names.cc b/chrome/common/pref_names.cc
index c49903662d46a706cacc06568d1755ebcf48c79c..692619e03ee4213a6db8fcc1a4618a58b1270a74 100644
--- a/chrome/common/pref_names.cc
+++ b/chrome/common/pref_names.cc
@@ -2071,6 +2071,10 @@ const char kServerBackedDeviceState[] = "server_backed_device_state";
 // since wallpaper was cached.
 const char kCustomizationDefaultWallpaperURL[] =
     "customization.default_wallpaper_url";
+
+// System uptime, when last logout started.
+// This is saved to file and cleared after chrome process starts.
+const char kLogoutStartedLast[] = "chromeos.logout-started";
 #endif
 
 // Whether there is a Flash version installed that supports clearing LSO data.