Add the CustomElementsRegistry interface behind the flag

third_party/WebKit/Source/core/frame/Window.idl

 /*
  * Copyright (C) 2006, 2007, 2008, 2009 Apple Inc. All rights reserved.
  * Copyright (C) 2011 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 68 matching lines @@
     [Measure] void print();
 
     [MeasureAs=UnprefixedRequestAnimationFrame] long requestAnimationFrame(FrameRequestCallback callback);
     void cancelAnimationFrame(long handle);
 
     [Measure, RuntimeEnabled=RequestIdleCallback] long requestIdleCallback(IdleRequestCallback callback, optional IdleRequestOptions options);
     [RuntimeEnabled=RequestIdleCallback] void cancelIdleCallback(long handle);
 
     [DoNotCheckSecurity, Custom, RaisesException] void postMessage(any message, DOMString targetOrigin, optional sequence<Transferable> transfer);
 
+    // Custom elements
+    // http://w3c.github.io/webcomponents/spec/custom/#custom-elements-api
+    [RuntimeEnabled=CustomElementsV1] readonly attribute CustomElementsRegistry customElements;
+
     // HTML obsolete features
     // https://html.spec.whatwg.org/#Window-partial
     [MeasureAs=WindowCaptureEvents] void captureEvents();
     [MeasureAs=WindowReleaseEvents] void releaseEvents();
 
     // CSS Object Model (CSSOM)
     // http://dev.w3.org/csswg/cssom/#extensions-to-the-window-interface
     // FIXME: The optional pseudoElt argument should have no default.
     [NewObject] CSSStyleDeclaration getComputedStyle(Element elt, optional DOMString? pseudoElt = null);
 
@@ skipping 98 matching lines @@
 // FIXME: make this typedef accurate once enough of http://crbug.com/240176
 // is in place.
 // FIXME: consider putting this typedef in an .idl file containing spec-wide
 // utility type definitions.
 typedef MessagePort Transferable;
 
 Window implements GlobalEventHandlers;
 Window implements WindowBase64;
 Window implements WindowEventHandlers;
 Window implements WindowTimers;