Move sync notifier contruction out of syncer thread.

chrome/browser/sync/notifier/sync_notifier.h

Index: chrome/browser/sync/notifier/sync_notifier.h
diff --git a/chrome/browser/sync/notifier/sync_notifier.h b/chrome/browser/sync/notifier/sync_notifier.h
index da15259c2b4146e0612388068623bb6cb6f663b2..5bf6c8c89820a96d8c7bd8d68eb9d498bdc1a3a5 100644
--- a/chrome/browser/sync/notifier/sync_notifier.h
+++ b/chrome/browser/sync/notifier/sync_notifier.h
@@ -5,6 +5,20 @@
 // Interface to the sync notifier, which is an object that receives
 // notifications when updates are available for a set of sync types.
 // All the observers are notified when such an event happens.
+//
+// A SyncNotifier must be destroyed on the same thread it was created on,
+// and all its methods must be called on the same thread (not necessarily
+// the one it was created on). If the methods thread is different from the
+// creation thread, then the methods thread must not exist when the SyncNotifier
+// is created and destroyed.
+//
+// In particular, the SyncNotifier will be created on the UI thread, the syncer
+// core thread will be created, the SyncNotifier will be used on that core
+// thread, the syncer core thread will be destroyed, and then the SyncNotifier
+// will be destroyed.
+//
+// TODO(akalin): Remove the code to deal with this situation once the syncer
+// core thread goes away.
 
 #ifndef CHROME_BROWSER_SYNC_NOTIFIER_SYNC_NOTIFIER_H_
 #define CHROME_BROWSER_SYNC_NOTIFIER_SYNC_NOTIFIER_H_