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

Source/core/rendering/RenderObject.h

Index: Source/core/rendering/RenderObject.h
diff --git a/Source/core/rendering/RenderObject.h b/Source/core/rendering/RenderObject.h
index 27427ae86f58fbf40e93069abae82dec1dbf8a52..60746515d1a3dd05bcfe3885ce4798a8df2b8371 100644
--- a/Source/core/rendering/RenderObject.h
+++ b/Source/core/rendering/RenderObject.h
@@ -599,6 +599,7 @@ public:
     // pseudo elements for which their parent node is returned.
     Node* generatingNode() const { return isPseudoElement() ? node()->parentOrShadowHostNode() : node(); }
 
+    // FIXME: This should return a reference.
     Document* document() const { return m_node->document(); }
     Frame* frame() const { return document()->frame(); }