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

Source/core/svg/SVGLengthContext.cpp

Index: Source/core/svg/SVGLengthContext.cpp
diff --git a/Source/core/svg/SVGLengthContext.cpp b/Source/core/svg/SVGLengthContext.cpp
index 7f214a3218685ca68cf1e7ec380f84ae21ee6c90..64a7a443c4fd351d379e5deaf2ba76a8c6dd9eff 100644
--- a/Source/core/svg/SVGLengthContext.cpp
+++ b/Source/core/svg/SVGLengthContext.cpp
@@ -294,7 +294,7 @@ bool SVGLengthContext::determineViewport(float& width, float& height) const
 
     // SVGLengthContext should NEVER be used to resolve width/height values for <svg> elements,
     // as they require special treatment, due the relationship with the CSS width/height properties.
-    ASSERT(m_context->document()->documentElement() != m_context);
+    ASSERT(m_context->document().documentElement() != m_context);
 
     // Take size from nearest viewport element.
     SVGElement* viewportElement = m_context->viewportElement();