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

Source/core/rendering/RenderObjectChildList.cpp

Index: Source/core/rendering/RenderObjectChildList.cpp
diff --git a/Source/core/rendering/RenderObjectChildList.cpp b/Source/core/rendering/RenderObjectChildList.cpp
index 2f3b47135a21c471abac58e9257f54ee29292c26..f5faaacbbf18c787820872f552bd5a95504664ba 100644
--- a/Source/core/rendering/RenderObjectChildList.cpp
+++ b/Source/core/rendering/RenderObjectChildList.cpp
@@ -109,7 +109,7 @@ RenderObject* RenderObjectChildList::removeChildNode(RenderObject* owner, Render
     if (!owner->documentBeingDestroyed())
         RenderCounter::rendererRemovedFromTree(oldChild);
 
-    if (AXObjectCache* cache = owner->document()->existingAXObjectCache())
+    if (AXObjectCache* cache = owner->document().existingAXObjectCache())
         cache->childrenChanged(owner);
 
     return oldChild;
@@ -161,7 +161,7 @@ void RenderObjectChildList::insertChildNode(RenderObject* owner, RenderObject* n
     if (!owner->normalChildNeedsLayout())
         owner->setChildNeedsLayout(); // We may supply the static position for an absolute positioned child.
 
-    if (AXObjectCache* cache = owner->document()->axObjectCache())
+    if (AXObjectCache* cache = owner->document().axObjectCache())
         cache->childrenChanged(owner);
 }