sync: SyncableService support for starting sync

sync/api/syncable_service.h

Index: sync/api/syncable_service.h
diff --git a/sync/api/syncable_service.h b/sync/api/syncable_service.h
index 13c282ef5a601484bf7a05656a441a33f2e5bb0a..13225ca98463c6f9c728d2b5b77b3d44c35bd73e 100644
--- a/sync/api/syncable_service.h
+++ b/sync/api/syncable_service.h
@@ -7,6 +7,7 @@
 
 #include <vector>
 
+#include "base/callback.h"
 #include "base/compiler_specific.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/memory/weak_ptr.h"
@@ -28,6 +29,15 @@ class SYNC_EXPORT SyncableService
     : public SyncChangeProcessor,
       public base::SupportsWeakPtr<SyncableService> {
  public:
+  // A StartSyncFlare is useful when your SyncableService has a need for sync
+  // to start ASAP, typically because a local change event has occurred but
+  // MergeDataAndStartSyncing hasn't been called yet, meaning you don't have a
+  // SyncChangeProcessor. The sync subsystem will respond soon after invoking
+  // Run() on your flare by calling MergeDataAndStartSyncing. The ModelType
+  // parameter is included so that the recieving end can track usage and timing
+  // statistics, make optimizations or tradeoffs by type, etc.
+  typedef base::Callback<void(ModelType)> StartSyncFlare;
+
   // Informs the service to begin syncing the specified synced datatype |type|.
   // The service should then merge |initial_sync_data| into it's local data,
   // calling |sync_processor|'s ProcessSyncChanges as necessary to reconcile the