Use refs for non-null GraphicsContext, Scrollbar, etc. in scrollbar related code

third_party/WebKit/Source/core/paint/FramePainter.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef FramePainter_h
 #define FramePainter_h
 
 #include "core/paint/PaintPhase.h"
 #include "platform/heap/Handle.h"
 
 namespace blink {
 
 class CullRect;
 class FrameView;
 class GraphicsContext;
 class IntRect;
 class Scrollbar;
 
 class FramePainter {
     STACK_ALLOCATED();
     WTF_MAKE_NONCOPYABLE(FramePainter);
 public:
     explicit FramePainter(const FrameView& frameView) : m_frameView(&frameView) { }
 
-    void paint(GraphicsContext*, const GlobalPaintFlags, const CullRect&);
-    void paintScrollbars(GraphicsContext*, const IntRect&);
-    void paintContents(GraphicsContext*, const GlobalPaintFlags, const IntRect&);
-    void paintScrollCorner(GraphicsContext*, const IntRect& cornerRect);
+    void paint(GraphicsContext&, const GlobalPaintFlags, const CullRect&);
+    void paintScrollbars(GraphicsContext&, const IntRect&);
+    void paintContents(GraphicsContext&, const GlobalPaintFlags, const IntRect&);
+    void paintScrollCorner(GraphicsContext&, const IntRect& cornerRect);
 
 private:
-    void paintScrollbar(GraphicsContext*, Scrollbar*, const IntRect&);
+    void paintScrollbar(GraphicsContext&, Scrollbar&, const IntRect&);
 
     const FrameView& frameView();
 
     RawPtrWillBeMember<const FrameView> m_frameView;
     static bool s_inPaintContents;
 };
 
 } // namespace blink
 
 #endif // FramePainter_h