[Sync] Replacing NULL with nullptr in code and null in comments for components/sync/

components/sync/driver/glue/sync_backend_host_mock.cc

Index: components/sync/driver/glue/sync_backend_host_mock.cc
diff --git a/components/sync/driver/glue/sync_backend_host_mock.cc b/components/sync/driver/glue/sync_backend_host_mock.cc
index f02c4a559a53f28c9f876f86944b380a39dfe449..bfe2afce586bce60638c050e23c586be7d423793 100644
--- a/components/sync/driver/glue/sync_backend_host_mock.cc
+++ b/components/sync/driver/glue/sync_backend_host_mock.cc
@@ -81,7 +81,7 @@ void SyncBackendHostMock::ActivateNonBlockingDataType(
 void SyncBackendHostMock::DeactivateNonBlockingDataType(ModelType type) {}
 
 UserShare* SyncBackendHostMock::GetUserShare() const {
-  return NULL;
+  return nullptr;
 }
 
 SyncBackendHost::Status SyncBackendHostMock::GetDetailedStatus() {
@@ -119,7 +119,7 @@ void SyncBackendHostMock::GetModelSafeRoutingInfo(
 void SyncBackendHostMock::FlushDirectory() const {}
 
 base::MessageLoop* SyncBackendHostMock::GetSyncLoopForTesting() {
-  return NULL;
+  return nullptr;
 }
 
 void SyncBackendHostMock::RefreshTypesForTest(ModelTypeSet types) {}