Editing: Let Selection.collapse with null clears selection.

Source/core/editing/DOMSelection.cpp

 /*
  * Copyright (C) 2007, 2009 Apple Inc. All rights reserved.
  * Copyright (C) 2012 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 179 matching lines @@
 
 int DOMSelection::rangeCount() const
 {
     if (!m_frame)
         return 0;
     return m_frame->selection().isNone() ? 0 : 1;
 }
 
 void DOMSelection::collapse(Node* node, int offset, ExceptionState& exceptionState)
 {
-    ASSERT(node);
     if (!m_frame)
         return;
 
+    if (!node) {
+        m_frame->selection().clear();
+        return;
+    }
+
     if (offset < 0) {
         exceptionState.throwDOMException(IndexSizeError, String::number(offset) + " is not a valid offset.");
         return;
     }
 
     if (!isValidForPosition(node))
         return;
     RefPtrWillBeRawPtr<Range> range = Range::create(node->document());
     range->setStart(node, offset, exceptionState);
     if (exceptionState.hadException())
@@ skipping 334 matching lines @@
     return node->document() == m_frame->document();
 }
 
 void DOMSelection::addConsoleError(const String& message)
 {
     if (m_treeScope)
         m_treeScope->document().addConsoleMessage(ConsoleMessage::create(JSMessageSource, ErrorMessageLevel, message));
 }
 
 } // namespace blink