Have DOMImplementation::document() return reference.

Source/core/dom/DOMImplementation.h

Index: Source/core/dom/DOMImplementation.h
diff --git a/Source/core/dom/DOMImplementation.h b/Source/core/dom/DOMImplementation.h
index 7a93cc00557938dea6b6f0dc86561d111bcaf9b2..55f846b67689041cc4089076b545a12a085f41ce 100644
--- a/Source/core/dom/DOMImplementation.h
+++ b/Source/core/dom/DOMImplementation.h
@@ -47,7 +47,7 @@ public:
 
     void ref() { m_document.ref(); }
     void deref() { m_document.deref(); }
-    Document* document() { return &m_document; }
+    Document& document() const { return m_document; }
 
     // DOM methods & attributes for DOMImplementation
     static bool hasFeature(const String& feature, const String& version);