Add adoptedCallback to work around Firefox Native CE bug (#34455)

See https://output.jsbin.com/yuwojok/29/quiet for a working demo of the bug. When we construct a CustomElement in one document, then insert it into another document (as we do with A4A ads), Firefox incorrectly resets the prototype of our CE instance to `HTMLElement.prototype`.

Luckily, we can fix this by listening for the `adoptedCallback`, which fires just after Firefox resets the prototype. If we detect a broken prototype chain, we can correct it before any other code can notice. See https://output.jsbin.com/datajos/5/quiet for a demo of the fix.

src/custom-element.js

@@ -98,7 +98,20 @@ export function createCustomElementClass(win, elementConnectedCallback) {
   );
   // It's necessary to create a subclass, because the same "base" class cannot
   // be registered to multiple custom elements.
-  class CustomAmpElement extends BaseCustomElement {}
+  class CustomAmpElement extends BaseCustomElement {
+    /**
+     * adoptedCallback is only called when using a Native implementation of Custom Elements V1.
+     * Our polyfill does not call this method.
+     */
+    adoptedCallback() {
+      // Work around an issue with Firefox changing the prototype of our
+      // already constructed element to the new document's HTMLElement.
+      if (Object.getPrototypeOf(this) !== customAmpElementProto) {
+        Object.setPrototypeOf(this, customAmpElementProto);
+      }
+    }
+  }
+  const customAmpElementProto = CustomAmpElement.prototype;
   return /** @type {typeof AmpElement} */ (CustomAmpElement);
 }