[IAP]Throw an error when the options.channel is undefined

If the options.channel is undefined, when passing it to navigator.iap.init()
method, the Promise should be rejected and an error named
"InvalidAccessError" is thrown also.

BUG=XWALK-6798

iap/iap.js

@@ -37,6 +37,8 @@ exports.init = function(options) {
   return new Promise(function(resolve, reject) {
     if (g_initialized)
       throw new DOMError("InvalidStateError");
+    if (typeof(options.channel) === "undefined")
+      throw new DOMError("InvalidAccessError");
     var resolveWrapper = function() {
       g_initialized = true;
       resolve();