Implemented the plumbing for paint worklets.

components/script/dom/webidls/PaintWorkletGlobalScope.webidl

@@ -0,0 +1,9 @@
+/* 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 http://mozilla.org/MPL/2.0/. */
+
+// https://drafts.css-houdini.org/css-paint-api/#paintworkletglobalscope
+[Global=(Worklet,PaintWorklet), Exposed=PaintWorklet]
+interface PaintWorkletGlobalScope : WorkletGlobalScope {
+    void registerPaint(DOMString name, VoidFunction paintCtor);
+};

components/script/dom/webidls/Window.webidl

@@ -204,3 +204,7 @@ partial interface Window {
    //readonly attribute EventSender eventSender;
 };
 
+// https://drafts.css-houdini.org/css-paint-api-1/#paint-worklet
+partial interface Window {
+    [SameObject] readonly attribute Worklet paintWorklet;
+};