Use method available in all browsers (#13804)

String.contains work in Firefox, not in Chrome or Opera

Change-Id: I074d3c79629fd699841d3b39a36620511233ce6c

WebContent/VAADIN/vaadinBootstrap.js

@@ -67,7 +67,7 @@
 			// "v-app-loading" we have only received the HTML 
 			// but not yet started the widget set
 			// (UIConnector removes the v-app-loading div).
-			if (className && className.contains("v-app-loading")) {
+			if (className && className.indexOf("v-app-loading") != -1) {
 				return false;
 			}
 		}