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

third_party/WebKit/Source/web/mac/WebScrollbarTheme.mm

 /*
  * Copyright (C) 2013 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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 29 matching lines @@
 
 namespace blink {
 
 static_assert(static_cast<NSScrollerStyle>(ScrollerStyleLegacy) == NSScrollerStyleLegacy, "ScrollerStyleLegacy must match NSScrollerStyleLegacy");
 static_assert(static_cast<NSScrollerStyle>(ScrollerStyleOverlay) == NSScrollerStyleOverlay, "ScrollerStyleOverlay must match NSScrollerStyleOverlay");
 
 void WebScrollbarTheme::updateScrollbarsWithNSDefaults(
     float initialButtonDelay, float autoscrollButtonDelay,
     ScrollerStyle preferredScrollerStyle, bool redraw, bool scrollAnimationEnabled, ScrollbarButtonsPlacement buttonPlacement)
 {
-    ScrollbarTheme* theme = ScrollbarTheme::theme();
-    if (theme->isMockTheme())
+    ScrollbarTheme& theme = ScrollbarTheme::theme();
+    if (theme.isMockTheme())
         return;
 
-    static_cast<ScrollbarThemeMacCommon*>(ScrollbarTheme::theme())->preferencesChanged(
+    static_cast<ScrollbarThemeMacCommon&>(theme).preferencesChanged(
         initialButtonDelay, autoscrollButtonDelay,
         static_cast<NSScrollerStyle>(preferredScrollerStyle),
         redraw, scrollAnimationEnabled, buttonPlacement);
 }
 
 } // namespace blink