Remove useless null checks from Node::document() call sites

Source/core/editing/VisibleUnits.cpp

 /*
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009 Apple 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
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 1311 matching lines @@
 
 bool isEndOfBlock(const VisiblePosition &pos)
 {
     return pos.isNotNull() && pos == endOfBlock(pos, CanCrossEditingBoundary);
 }
 
 // ---------
 
 VisiblePosition startOfDocument(const Node* node)
 {
-    if (!node || !node->document() || !node->document()->documentElement())
+    if (!node || !node->document()->documentElement())
         return VisiblePosition();
 
     return VisiblePosition(firstPositionInNode(node->document()->documentElement()), DOWNSTREAM);
 }
 
 VisiblePosition startOfDocument(const VisiblePosition &c)
 {
     return startOfDocument(c.deepEquivalent().deprecatedNode());
 }
 
 VisiblePosition endOfDocument(const Node* node)
 {
-    if (!node || !node->document() || !node->document()->documentElement())
+    if (!node || !node->document()->documentElement())
         return VisiblePosition();
 
     Element* doc = node->document()->documentElement();
     return VisiblePosition(lastPositionInNode(doc), DOWNSTREAM);
 }
 
 VisiblePosition endOfDocument(const VisiblePosition &c)
 {
     return endOfDocument(c.deepEquivalent().deprecatedNode());
 }
@@ skipping 51 matching lines @@
 {
     return direction == LTR ? logicalStartOfLine(c) : logicalEndOfLine(c);
 }
 
 VisiblePosition rightBoundaryOfLine(const VisiblePosition& c, TextDirection direction)
 {
     return direction == LTR ? logicalEndOfLine(c) : logicalStartOfLine(c);
 }
 
 }