Add support for temporarily disabling sync.

chrome/browser/sync/sync_prefs.cc

Index: chrome/browser/sync/sync_prefs.cc
diff --git a/chrome/browser/sync/sync_prefs.cc b/chrome/browser/sync/sync_prefs.cc
index 18d745621cba18bdc73d2219cfadbe4a2e956914..2f01afc79f4ccf68740aa07b3ff2b9112f02533c 100644
--- a/chrome/browser/sync/sync_prefs.cc
+++ b/chrome/browser/sync/sync_prefs.cc
@@ -86,6 +86,13 @@ void SyncPrefs::SetStartSuppressed(bool is_suppressed) {
   pref_service_->ScheduleSavePersistentPrefs();
 }
 
+std::string SyncPrefs::GetGoogleServicesUsername() const {
+  DCHECK(non_thread_safe_.CalledOnValidThread());
+  return
+      pref_service_ ?
+      pref_service_->GetString(prefs::kGoogleServicesUsername) : "";
+}
+
 base::Time SyncPrefs::GetLastSyncedTime() const {
   DCHECK(non_thread_safe_.CalledOnValidThread());
   return