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

chrome/common/chrome_switches.cc

Index: chrome/common/chrome_switches.cc
===================================================================
--- chrome/common/chrome_switches.cc	(revision 89823)
+++ chrome/common/chrome_switches.cc	(working copy)
@@ -373,6 +373,10 @@
 // for more background.
 const char kEnableAuthNegotiatePort[]       = "enable-auth-negotiate-port";
 
+// Disable both pre- and auto-login for services attached to a GAIA account
+// even when the GAIA credentials are available through sync.
+const char kDisableAutoLogin[]              = "disable-auto-login";
+
 // This flag enables UI for clearing server data.  Temporarily in place
 // until there's a server endpoint deployed.
 const char kEnableClearServerData[]         = "enable-clear-server-data";