Editing: Let Selection.collapse with null clears selection.

Source/core/editing/Selection.idl

 /*
  * Copyright (C) 2007 Apple Inc.  All rights reserved.
  * Copyright (C) 2009 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.
@@ skipping 27 matching lines @@
 [
     ImplementedAs=DOMSelection,
     WillBeGarbageCollected,
 ] interface Selection {
     readonly attribute Node anchorNode;
     readonly attribute long anchorOffset;
     readonly attribute Node focusNode;
     readonly attribute long focusOffset;
 
     readonly attribute boolean isCollapsed;
-    [RaisesException, TypeChecking=Interface] void collapse(Node node, optional long offset = 0);
+    [RaisesException] void collapse(Node? node, optional long offset = 0);
     [RaisesException] void collapseToStart();
     [RaisesException] void collapseToEnd();
 
     // We mark offset as optional, defaulting to 0; this differs from spec.
     // http://crbug.com/384966
     [RaisesException, TypeChecking=Interface] void extend(Node node, optional long offset = 0);
 
     [RaisesException] void selectAllChildren([Default=Undefined] optional Node node);
     [CustomElementCallbacks] void deleteFromDocument();
 
@@ skipping 19 matching lines @@
     // IE's type accessor returns "none", "text" and "control"
     [MeasureAs=SelectionType] readonly attribute DOMString type;
 
     [MeasureAs=SelectionModify] void modify([Default=Undefined] optional DOMString alter,
                                             [Default=Undefined] optional DOMString direction,
                                             [Default=Undefined] optional DOMString granularity);
     [MeasureAs=SelectionSetBaseAndExtent, RaisesException] void setBaseAndExtent([Default=Undefined] optional Node baseNode,
                                                                                  [Default=Undefined] optional long baseOffset,
                                                                                  [Default=Undefined] optional Node extentNode,
                                                                                  [Default=Undefined] optional long extentOffset);
-    [ImplementedAs=collapse, MeasureAs=SelectionSetPosition, RaisesException, TypeChecking=Interface] void setPosition(Node node,
-                                                                                                                       optional long offset = 0);
+    [ImplementedAs=collapse, MeasureAs=SelectionSetPosition, RaisesException] void setPosition(Node? node, optional long offset = 0);
 
     // IE extensions
     // http://msdn.microsoft.com/en-us/library/ms535869(VS.85).aspx
     [MeasureAs=SelectionEmpty] void empty();
 };