Fix storybook by shimming MV3 method (#15749)

.storybook/__mocks__/webextension-polyfill.js

@@ -1,4 +1,8 @@
 module.exports = {
-    runtime: {},
+  runtime: {
+    getManifest: () => {
+      return { manifest_version: 2 };
+    }
+  },
 };