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..ec2b8b02c614620b048a64c471217f4d49b93a40 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"
@@ -21,6 +22,16 @@ namespace syncer {
 
 class SyncErrorFactory;
 
+// Used as a parameter to InjectStartSyncFlare. The 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;
+
 // TODO(zea): remove SupportsWeakPtr in favor of having all SyncableService
 // implementers provide a way of getting a weak pointer to themselves.
 // See crbug.com/100114.
@@ -28,6 +39,10 @@ class SYNC_EXPORT SyncableService
     : public SyncChangeProcessor,
       public base::SupportsWeakPtr<SyncableService> {
  public:
+  // Provides a StartSyncFlare to the SyncableService.
+  // TODO(tim): BUG 80194. Make pure virtual.
+  virtual void InjectStartSyncFlare(const StartSyncFlare& flare);
+
   // 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