dom: Stub out the IntersectionObserver interface (#33989)

This is the first step toward implementing the IntersectionObserver
interface. It adds stubs which are exposed when a preference is turned
on. This is enough to get some sites with `IntersectionObserver` to
start working.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

components/script/dom/webidls/IntersectionObserver.webidl

@@ -0,0 +1,32 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+// https://w3c.github.io/IntersectionObserver/#intersection-observer-interface
+
+callback IntersectionObserverCallback =
+  undefined (sequence<IntersectionObserverEntry> entries, IntersectionObserver observer);
+
+dictionary IntersectionObserverInit {
+  (Element or Document)?  root = null;
+  DOMString rootMargin;
+  DOMString scrollMargin;
+  (double or sequence<double>) threshold;
+  long delay;
+  boolean trackVisibility = false;
+};
+
+[Pref="dom.intersection_observer.enabled", Exposed=(Window)]
+interface IntersectionObserver {
+  constructor(IntersectionObserverCallback callback, optional IntersectionObserverInit options = {});
+  readonly attribute (Element or Document)? root;
+  readonly attribute DOMString rootMargin;
+  readonly attribute DOMString scrollMargin;
+  readonly attribute /* FrozenArray<double> */ any thresholds;
+  readonly attribute long delay;
+  readonly attribute boolean trackVisibility;
+  undefined observe(Element target);
+  undefined unobserve(Element target);
+  undefined disconnect();
+  sequence<IntersectionObserverEntry> takeRecords();
+};

components/script/dom/webidls/IntersectionObserverEntry.webidl

@@ -0,0 +1,29 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+// https://w3c.github.io/IntersectionObserver/#intersection-observer-entry
+
+[Pref="dom.intersection_observer.enabled", Exposed=(Window)]
+interface IntersectionObserverEntry {
+  constructor(IntersectionObserverEntryInit intersectionObserverEntryInit);
+  readonly attribute DOMHighResTimeStamp time;
+  readonly attribute DOMRectReadOnly? rootBounds;
+  readonly attribute DOMRectReadOnly boundingClientRect;
+  readonly attribute DOMRectReadOnly intersectionRect;
+  readonly attribute boolean isIntersecting;
+  readonly attribute boolean isVisible;
+  readonly attribute double intersectionRatio;
+  readonly attribute Element target;
+};
+
+dictionary IntersectionObserverEntryInit {
+  required DOMHighResTimeStamp time;
+  required DOMRectInit rootBounds;
+  required DOMRectInit boundingClientRect;
+  required DOMRectInit intersectionRect;
+  required boolean isIntersecting;
+  required boolean isVisible;
+  required double intersectionRatio;
+  required Element target;
+};