Set up fallback for iframeUrl and apiUrl.

javascript/src/viamapi-client.js

@@ -10,7 +10,13 @@ const Penpal = require('penpal').default;
  */
 function setupViamAPI(divId, methods, iframeUrl, apiUrl) {
   if (!apiUrl) {
-    return Promise.reject('API Host not specified.');
+    apiUrl = `${window.location.origin}/api/`;
+    console.warn(`API host URL not specified. Fall back to ${apiUrl}`); // eslint-disable-line no-console
+  }
+
+  if (!iframeUrl) {
+    iframeUrl = `${window.location.origin}/vcl/js/iframe`;
+    console.warn(`Iframe URL not specified. Fall back to ${iframeUrl}`); // eslint-disable-line no-console
   }
 
   const connection = Penpal.connectToChild({