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

Source/core/rendering/RenderTheme.cpp

Index: Source/core/rendering/RenderTheme.cpp
diff --git a/Source/core/rendering/RenderTheme.cpp b/Source/core/rendering/RenderTheme.cpp
index 22db462a15da33e5e3a50897ce9bb7a039e6330c..60eb7bbd384123d80914501725eec4db9d49332c 100644
--- a/Source/core/rendering/RenderTheme.cpp
+++ b/Source/core/rendering/RenderTheme.cpp
@@ -803,7 +803,7 @@ bool RenderTheme::isActive(const RenderObject* o) const
     if (!node)
         return false;
 
-    Page* page = node->document()->page();
+    Page* page = node->document().page();
     if (!page)
         return false;
 
@@ -839,9 +839,9 @@ bool RenderTheme::isFocused(const RenderObject* o) const
         return false;
 
     node = node->focusDelegate();
-    Document* document = node->document();
-    Frame* frame = document->frame();
-    return node == document->focusedElement() && node->shouldHaveFocusAppearance() && frame && frame->selection()->isFocusedAndActive();
+    Document& document = node->document();
+    Frame* frame = document.frame();
+    return node == document.focusedElement() && node->shouldHaveFocusAppearance() && frame && frame->selection()->isFocusedAndActive();
 }
 
 bool RenderTheme::isPressed(const RenderObject* o) const