Have Node::document() return a reference instead of a pointer

Source/core/rendering/RenderReplaced.cpp

Index: Source/core/rendering/RenderReplaced.cpp
diff --git a/Source/core/rendering/RenderReplaced.cpp b/Source/core/rendering/RenderReplaced.cpp
index 328ad77c62e4503dc83bc4f1202f1b16696b302e..596957a289d2e26162a9049c00bbd72a0cbbacfc 100644
--- a/Source/core/rendering/RenderReplaced.cpp
+++ b/Source/core/rendering/RenderReplaced.cpp
@@ -128,7 +128,7 @@ void RenderReplaced::paint(PaintInfo& paintInfo, const LayoutPoint& paintOffset)
     if (!paintInfo.shouldPaintWithinRoot(this))
         return;
 
-    bool drawSelectionTint = selectionState() != SelectionNone && !document()->printing();
+    bool drawSelectionTint = selectionState() != SelectionNone && !document().printing();
     if (paintInfo.phase == PaintPhaseSelection) {
         if (selectionState() == SelectionNone)
             return;