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

Source/core/html/shadow/SliderThumbElement.cpp

Index: Source/core/html/shadow/SliderThumbElement.cpp
diff --git a/Source/core/html/shadow/SliderThumbElement.cpp b/Source/core/html/shadow/SliderThumbElement.cpp
index eea1a7addd0af93a146fe09bd613b46e65af8c92..d6751cb241becf57d4269faaf292c13e131c45c2 100644
--- a/Source/core/html/shadow/SliderThumbElement.cpp
+++ b/Source/core/html/shadow/SliderThumbElement.cpp
@@ -312,7 +312,7 @@ void SliderThumbElement::setPositionFromPoint(const LayoutPoint& point)
 
 void SliderThumbElement::startDragging()
 {
-    if (Frame* frame = document()->frame()) {
+    if (Frame* frame = document().frame()) {
         frame->eventHandler()->setCapturingMouseEventsNode(this);
         m_inDragMode = true;
     }
@@ -323,7 +323,7 @@ void SliderThumbElement::stopDragging()
     if (!m_inDragMode)
         return;
 
-    if (Frame* frame = document()->frame())
+    if (Frame* frame = document().frame())
         frame->eventHandler()->setCapturingMouseEventsNode(0);
     m_inDragMode = false;
     if (renderer())
@@ -389,7 +389,7 @@ bool SliderThumbElement::willRespondToMouseClickEvents()
 void SliderThumbElement::detach(const AttachContext& context)
 {
     if (m_inDragMode) {
-        if (Frame* frame = document()->frame())
+        if (Frame* frame = document().frame())
             frame->eventHandler()->setCapturingMouseEventsNode(0);
     }
     HTMLDivElement::detach(context);