Introduce composed tree version of startOfDocument()

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 1539 matching lines @@
     return pos.isNotNull() && pos.deepEquivalent() == startOfBlock(pos, CanCrossEditingBoundary).deepEquivalent();
 }
 
 bool isEndOfBlock(const VisiblePosition& pos)
 {
     return pos.isNotNull() && pos.deepEquivalent() == endOfBlock(pos, CanCrossEditingBoundary).deepEquivalent();
 }
 
 // ---------
 
-VisiblePosition startOfDocument(const Node* node)
+template <typename Strategy>
+static VisiblePositionTemplate<Strategy> startOfDocumentAlgorithm(const VisiblePositionTemplate<Strategy>& visiblePosition)
 {
+    Node* node = visiblePosition.deepEquivalent().anchorNode();
     if (!node || !node->document().documentElement())
-        return VisiblePosition();
+        return VisiblePositionTemplate<Strategy>();
 
-    return createVisiblePosition(firstPositionInNode(node->document().documentElement()));
+    return createVisiblePosition(PositionAlgorithm<Strategy>::firstPositionInNode(node->document().documentElement()));
 }
 
 VisiblePosition startOfDocument(const VisiblePosition& c)
 {
-    return startOfDocument(c.deepEquivalent().anchorNode());
+    return startOfDocumentAlgorithm<EditingStrategy>(c);
+}
+
+VisiblePositionInComposedTree startOfDocument(const VisiblePositionInComposedTree& c)
+{
+    return startOfDocumentAlgorithm<EditingInComposedTreeStrategy>(c);
 }
 
 template <typename Strategy>
 static VisiblePositionTemplate<Strategy> endOfDocumentAlgorithm(const VisiblePositionTemplate<Strategy>& visiblePosition)
 {
     Node* node = visiblePosition.deepEquivalent().anchorNode();
     if (!node || !node->document().documentElement())
         return VisiblePositionTemplate<Strategy>();
 
     Element* doc = node->document().documentElement();
@@ skipping 1469 matching lines @@
 {
     return previousPositionOfAlgorithm<EditingStrategy>(visiblePosition, rule);
 }
 
 VisiblePositionInComposedTree previousPositionOf(const VisiblePositionInComposedTree& visiblePosition, EditingBoundaryCrossingRule rule)
 {
     return previousPositionOfAlgorithm<EditingInComposedTreeStrategy>(visiblePosition, rule);
 }
 
 } // namespace blink