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

third_party/WebKit/Source/core/layout/LayoutScrollbarPart.cpp

 /*
  * Copyright (C) 2008 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 102 matching lines @@
     } else {
         setWidth(m_scrollbar->width());
         computeScrollbarHeight();
     }
 }
 
 static int calcScrollbarThicknessUsing(SizeType sizeType, const Length& length, int containingLength)
 {
     if (!length.isIntrinsicOrAuto() || (sizeType == MinSize && length.isAuto()))
         return minimumValueForLength(length, containingLength);
-    return ScrollbarTheme::theme()->scrollbarThickness();
+    return ScrollbarTheme::theme().scrollbarThickness();
 }
 
 void LayoutScrollbarPart::computeScrollbarWidth()
 {
     if (!m_scrollbar->owningLayoutObject())
         return;
     // FIXME: We are querying layout information but nothing guarantees that it's up-to-date, especially since we are called at style change.
     // FIXME: Querying the style's border information doesn't work on table cells with collapsing borders.
     int visibleSize = m_scrollbar->owningLayoutObject()->size().width() - m_scrollbar->owningLayoutObject()->style()->borderLeftWidth() - m_scrollbar->owningLayoutObject()->style()->borderRightWidth();
     int w = calcScrollbarThicknessUsing(MainOrPreferredSize, style()->width(), visibleSize);
@@ skipping 75 matching lines @@
             frameView->setScrollCornerNeedsPaintInvalidation();
             return;
         }
     }
 
     // This LayoutScrollbarPart belongs to a PaintLayerScrollableArea.
     toLayoutBox(parent())->scrollableArea()->setScrollCornerNeedsPaintInvalidation();
 }
 
 }