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

third_party/WebKit/Source/platform/scroll/ScrollbarThemeMock.h

 /*
  * Copyright (C) 2011 Apple 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
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 22 matching lines @@
 // Scrollbar theme used in image snapshots, to eliminate appearance differences between platforms.
 class PLATFORM_EXPORT ScrollbarThemeMock : public ScrollbarTheme {
 public:
     static void setShouldRepaintAllPartsOnInvalidation(bool);
 
     int scrollbarThickness(ScrollbarControlSize = RegularScrollbar) override;
     bool usesOverlayScrollbars() const override;
 
 protected:
     bool shouldRepaintAllPartsOnInvalidation() const override;
-    bool hasButtons(const ScrollbarThemeClient*) override { return false; }
-    bool hasThumb(const ScrollbarThemeClient*) override { return true; }
+    bool hasButtons(const ScrollbarThemeClient&) override { return false; }
+    bool hasThumb(const ScrollbarThemeClient&) override { return true; }
 
-    IntRect backButtonRect(const ScrollbarThemeClient*, ScrollbarPart, bool /*painting*/ = false) override { return IntRect(); }
-    IntRect forwardButtonRect(const ScrollbarThemeClient*, ScrollbarPart, bool /*painting*/ = false) override { return IntRect(); }
-    IntRect trackRect(const ScrollbarThemeClient*, bool painting = false) override;
+    IntRect backButtonRect(const ScrollbarThemeClient&, ScrollbarPart, bool /*painting*/ = false) override { return IntRect(); }
+    IntRect forwardButtonRect(const ScrollbarThemeClient&, ScrollbarPart, bool /*painting*/ = false) override { return IntRect(); }
+    IntRect trackRect(const ScrollbarThemeClient&, bool painting = false) override;
 
-    void paintTrackBackground(GraphicsContext*, const ScrollbarThemeClient*, const IntRect&) override;
-    void paintThumb(GraphicsContext*, const ScrollbarThemeClient*, const IntRect&) override;
+    void paintTrackBackground(GraphicsContext&, const ScrollbarThemeClient&, const IntRect&) override;
+    void paintThumb(GraphicsContext&, const ScrollbarThemeClient&, const IntRect&) override;
 
 private:
     bool isMockTheme() const final { return true; }
 };
 
 }
 #endif // ScrollbarThemeMock_h