Fix error: "Can't find variable: TextDecoder" (#6011)

* Update amp-ad-network-doubleclick-impl.md

* Reverting doc changes

The previous change was made in error (wrong branch).

* Initial changes, need to update tests

* Start fixing tests

* Fix unit test coverage

* revert a4a examples

* Include self (window) when verifying TextDecoder/TextEncoder existence

* fix upstream conflict

* PR feedback

* PR feedback

src/utils/bytes.js

@@ -23,7 +23,7 @@ import {dev} from '../log';
  * @return {!Promise<string>}
  */
 export function utf8Decode(bytes) {
-  if (TextDecoder) {
+  if (typeof TextDecoder !== 'undefined') {
     return Promise.resolve(new TextDecoder('utf-8').decode(bytes));
   }
   return new Promise((resolve, reject) => {
@@ -44,7 +44,7 @@ export function utf8Decode(bytes) {
  * @return {!Promise<!Uint8Array>}
  */
 export function utf8Encode(string) {
-  if (TextEncoder) {
+  if (typeof TextEncoder !== 'undefined') {
     return Promise.resolve(new TextEncoder('utf-8').encode(string));
   }
   return new Promise((resolve, reject) => {