Enable strict type checking for Selection.selectAllChildren()

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 475 matching lines @@
         || (Range::compareBoundaryPoints(parentNode, nodeIndex + 1, selectedRange->startContainer(), selectedRange->startOffset(), exceptionState) < 0 && !exceptionState.hadException());
     ASSERT(!exceptionState.hadException());
     if (nodeFullyUnselected)
         return false;
 
     return allowPartial || n->isTextNode();
 }
 
 void DOMSelection::selectAllChildren(Node* n, ExceptionState& exceptionState)
 {
-    if (!n)
-        return;
+    ASSERT(n);
 
     // This doesn't (and shouldn't) select text node characters.
     setBaseAndExtent(n, 0, n, n->countChildren(), exceptionState);
 }
 
 String DOMSelection::toString()
 {
     if (!m_frame)
         return String();
 
@@ skipping 52 matching lines @@
         m_treeScope->document().addConsoleMessage(ConsoleMessage::create(JSMessageSource, ErrorMessageLevel, message));
 }
 
 void DOMSelection::trace(Visitor* visitor)
 {
     visitor->trace(m_treeScope);
     DOMWindowProperty::trace(visitor);
 }
 
 } // namespace blink