Revert the revert. This change was not the cause of the failures, so relanding.

chrome/browser/sync/engine/all_status.cc

Index: chrome/browser/sync/engine/all_status.cc
===================================================================
--- chrome/browser/sync/engine/all_status.cc	(revision 110530)
+++ chrome/browser/sync/engine/all_status.cc	(working copy)
@@ -188,6 +188,11 @@
   status_.crypto_has_pending_keys = has_pending_keys;
 }
 
+void AllStatus::SetUniqueId(const std::string& guid) {
+  ScopedStatusLock lock(this);
+  status_.unique_id = guid;
+}
+
 ScopedStatusLock::ScopedStatusLock(AllStatus* allstatus)
     : allstatus_(allstatus) {
   allstatus->mutex_.Acquire();