Implement Invalidator::Acknowledge

sync/internal_api/test/fake_sync_manager.cc

Index: sync/internal_api/test/fake_sync_manager.cc
diff --git a/sync/internal_api/test/fake_sync_manager.cc b/sync/internal_api/test/fake_sync_manager.cc
index bc9c4dca810957b6ca1f0d1cfc1b237dd0a6183a..57c6c776acadf89b9d0034989080d1991531a1c6 100644
--- a/sync/internal_api/test/fake_sync_manager.cc
+++ b/sync/internal_api/test/fake_sync_manager.cc
@@ -172,6 +172,11 @@ void FakeSyncManager::UnregisterInvalidationHandler(
   registrar_.UnregisterHandler(handler);
 }
 
+void FakeSyncManager::AcknowledgeInvalidation(const invalidation::ObjectId& id,
+                                              const AckHandle& ack_handle) {
+  // Do nothing.
+}
+
 void FakeSyncManager::StartSyncingNormally(
       const ModelSafeRoutingInfo& routing_info) {
   // Do nothing.