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

third_party/WebKit/Source/core/paint/ScrollableAreaPainter.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 ScrollableAreaPainter_h
 #define ScrollableAreaPainter_h
 
 #include "platform/heap/Handle.h"
 
 namespace blink {
 
 class CullRect;
 class GraphicsContext;
 class IntPoint;
 class IntRect;
 class PaintLayerScrollableArea;
 
 class ScrollableAreaPainter {
     STACK_ALLOCATED();
     WTF_MAKE_NONCOPYABLE(ScrollableAreaPainter);
 public:
     explicit ScrollableAreaPainter(PaintLayerScrollableArea& paintLayerScrollableArea) : m_scrollableArea(&paintLayerScrollableArea) { }
 
-    void paintResizer(GraphicsContext*, const IntPoint& paintOffset, const CullRect&);
-    void paintOverflowControls(GraphicsContext*, const IntPoint& paintOffset, const CullRect&, bool paintingOverlayControls);
-    void paintScrollCorner(GraphicsContext*, const IntPoint&, const CullRect&);
+    void paintResizer(GraphicsContext&, const IntPoint& paintOffset, const CullRect&);
+    void paintOverflowControls(GraphicsContext&, const IntPoint& paintOffset, const CullRect&, bool paintingOverlayControls);
+    void paintScrollCorner(GraphicsContext&, const IntPoint&, const CullRect&);
 
 private:
-    void drawPlatformResizerImage(GraphicsContext*, IntRect resizerCornerRect);
+    void drawPlatformResizerImage(GraphicsContext&, IntRect resizerCornerRect);
     bool overflowControlsIntersectRect(const CullRect&) const;
 
     PaintLayerScrollableArea& scrollableArea() const;
 
     RawPtrWillBeMember<PaintLayerScrollableArea> m_scrollableArea;
 };
 
 } // namespace blink
 
 #endif // ScrollableAreaPainter_h