amp-action: Support whitelist lookup in AmpDocShadow (#26684)

* amp-action: Support whitelist lookup in ShadowRoot

New getMetaByName method supports retrieving meta content values in
shadow docs as well.

* amp-action: Remove unnecessary doc check

It is safe to assume that either document.head is available if
AmpDocSingle/AmpDocFie, or that AmpDoc.meta_ is non-null if
AmpDocShadow.

src/service/action-impl.js

@@ -807,17 +807,12 @@ export class ActionService {
    * @private
    */
   queryWhitelist_() {
-    const {head} = this.ampdoc.getRootNode();
-    if (!head) {
-      return null;
-    }
-    const meta = head.querySelector('meta[name="amp-action-whitelist"]');
-    if (!meta) {
+    const actionWhitelist = this.ampdoc.getMetaByName('amp-action-whitelist');
+    if (actionWhitelist === null) {
       return null;
     }
     return (
-      meta
-        .getAttribute('content')
+      actionWhitelist
         .split(',')
         // Turn an empty string whitelist into an empty array, otherwise the
         // parse error in the mapper below would trigger.

test/unit/test-action.js

@@ -39,7 +39,14 @@ import {whenCalled} from '../../testing/test-helper.js';
  */
 function actionService() {
   const win = {
-    document: {body: {}},
+    document: {
+      body: {},
+      head: {
+        nodeType: /* ELEMENT */ 1,
+        querySelector: () => null,
+        querySelectorAll: () => [],
+      },
+    },
     __AMP_SERVICES: {
       vsync: {obj: {}},
     },