[nwd] Fixed the trustedTypes variable identification bug

There is a small bug with the the "trustedTypes" variable which is not found as I was missing the "self." statement in index.js.

Change-Id: Id6976aed52943c8435857e46c62719fe54ef73f3

index.js

diff --git a/index.js b/index.js
index ed5bb1d..ae9515f 100644
--- a/index.js
+++ b/index.js
@@ -7,10 +7,10 @@
 var getProto = Object.getPrototypeOf;
 
 class TrustedFunction {
-  static policy = trustedTypes.createPolicy('TrustedFunctionWorkaround', {
+  static policy = self.trustedTypes.createPolicy('TrustedFunctionWorkaround', {
     createScript: (_, ...args) => {
        args.forEach( (arg) => {
-         if (!trustedTypes.isScript(arg)) {
+         if (!self.trustedTypes.isScript(arg)) {
            throw new Error("TrustedScripts only, please");
          }
        });