When a user logs into sync, the appropriate cookies are retrieved so that

chrome/browser/about_flags.cc

Index: chrome/browser/about_flags.cc
===================================================================
--- chrome/browser/about_flags.cc	(revision 88668)
+++ chrome/browser/about_flags.cc	(working copy)
@@ -331,6 +331,13 @@
     kOsAll,
     SINGLE_VALUE_TYPE(switches::kPreloadInstantSearch)
   },
+  {
+    "disable-auto-login",
+    IDS_FLAGS_AUTO_LOGIN_NAME,
+    IDS_FLAGS_AUTO_LOGIN_DESCRIPTION,
+    kOsMac | kOsWin | kOsLinux,
+    SINGLE_VALUE_TYPE(switches::kDisableAutoLogin)
+  },
 };
 
 const Experiment* experiments = kExperiments;