change from method reference to lambda since java 8 complained about …

…raw type

xds/src/main/java/io/grpc/xds/XdsDependencyManager.java

@@ -308,7 +308,7 @@ XdsConfig buildConfig() {
     List<String> topLevelClusters =
         cdsWatchers.values().stream()
             .filter(XdsDependencyManager::isTopLevelCluster)
-            .map(XdsWatcherBase::resourceName)
+            .map(w -> w.resourceName())
             .collect(Collectors.toList());
 
     // Flatten multi-level aggregates into lists of leaf clusters