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

third_party/WebKit/Source/platform/exported/WebScrollbarThemeClientImpl.h

 /*
  * Copyright (C) 2012 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
  * 2.  Redistributions in binary form must reproduce the above copyright
@@ skipping 21 matching lines @@
 #include "wtf/Noncopyable.h"
 
 namespace blink {
 
 // Adapts a WebScrollbar to the ScrollbarThemeClient interface
 class PLATFORM_EXPORT WebScrollbarThemeClientImpl : public ScrollbarThemeClient {
     WTF_MAKE_NONCOPYABLE(WebScrollbarThemeClientImpl);
 public:
     // Caller must retain ownership of this pointer and ensure that its lifetime
     // exceeds this instance.
-    WebScrollbarThemeClientImpl(WebScrollbar*);
+    WebScrollbarThemeClientImpl(WebScrollbar&);
     ~WebScrollbarThemeClientImpl() override;
 
     // Implement ScrollbarThemeClient interface
     int x() const override;
     int y() const override;
     int width() const override;
     int height() const override;
     IntSize size() const override;
     IntPoint location() const override;
     Widget* parent() const override;
@@ skipping 26 matching lines @@
     void setIsAlphaLocked(bool) override;
     float elasticOverscroll() const override;
     void setElasticOverscroll(float) override;
     bool trackNeedsRepaint() const override;
     void setTrackNeedsRepaint(bool) override;
     bool thumbNeedsRepaint() const override;
     void setThumbNeedsRepaint(bool) override;
     String debugName() const final;
 
 private:
-    WebScrollbar* m_scrollbar;
+    WebScrollbar& m_scrollbar;
 };
 
 } // namespace blink
 
 #endif // WebScrollbarThemeClientImpl_h