Move implementation of Range::firstNode and Range::pastLastNode to Position

Source/core/dom/Range.cpp

Index: Source/core/dom/Range.cpp
diff --git a/Source/core/dom/Range.cpp b/Source/core/dom/Range.cpp
index dffd506bc95c1120b7bb0e26b1ce20e35a1bc6cc..a27e461926f28db32139dffb77454fcbbe4abe0f 100644
--- a/Source/core/dom/Range.cpp
+++ b/Source/core/dom/Range.cpp
@@ -1414,13 +1414,7 @@ void Range::checkExtractPrecondition(ExceptionState& exceptionState)
 
 Node* Range::firstNode() const
 {
-    if (m_start.container()->offsetInCharacters())
-        return m_start.container();
-    if (Node* child = NodeTraversal::childAt(*m_start.container(), m_start.offset()))
-        return child;
-    if (!m_start.offset())
-        return m_start.container();
-    return NodeTraversal::nextSkippingChildren(*m_start.container());
+    return startPosition().toOffsetInAnchor().nodeAsRangeFirstNode();

[sof, 2015/05/05 08:23:23]

Is this a meaning-preserving transformation - what if the start node doesn't
have an anchor?

All uses of Range::firstNode() currently crash as a result - addressing via
https://codereview.chromium.org/1121633004/

 }
 
 ShadowRoot* Range::shadowRoot() const
@@ -1430,11 +1424,7 @@ ShadowRoot* Range::shadowRoot() const
 
 Node* Range::pastLastNode() const
 {
-    if (m_end.container()->offsetInCharacters())
-        return NodeTraversal::nextSkippingChildren(*m_end.container());
-    if (Node* child = NodeTraversal::childAt(*m_end.container(), m_end.offset()))
-        return child;
-    return NodeTraversal::nextSkippingChildren(*m_end.container());
+    return endPosition().toOffsetInAnchor().nodeAsRangePastLastNode();
 }
 
 IntRect Range::boundingBox() const