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

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

 /*
  * Copyright (C) 2008, 2009 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 224 matching lines @@
 {
     if (partType == NoPart)
         return;
 
     RefPtr<ComputedStyle> partStyle = !destroy ? getScrollbarPseudoStyle(partType,  pseudoForScrollbarPart(partType)) : PassRefPtr<ComputedStyle>(nullptr);
 
     bool needLayoutObject = !destroy && partStyle && partStyle->display() != NONE;
 
     if (needLayoutObject && partStyle->display() != BLOCK) {
         // See if we are a button that should not be visible according to OS settings.
-        ScrollbarButtonsPlacement buttonsPlacement = theme()->buttonsPlacement();
+        ScrollbarButtonsPlacement buttonsPlacement = theme().buttonsPlacement();
         switch (partType) {
         case BackButtonStartPart:
             needLayoutObject = (buttonsPlacement == ScrollbarButtonsPlacementSingle || buttonsPlacement == ScrollbarButtonsPlacementDoubleStart
                 || buttonsPlacement == ScrollbarButtonsPlacementDoubleBoth);
             break;
         case ForwardButtonStartPart:
             needLayoutObject = (buttonsPlacement == ScrollbarButtonsPlacementDoubleStart || buttonsPlacement == ScrollbarButtonsPlacementDoubleBoth);
             break;
         case BackButtonEndPart:
             needLayoutObject = (buttonsPlacement == ScrollbarButtonsPlacementDoubleEnd || buttonsPlacement == ScrollbarButtonsPlacementDoubleBoth);
@@ skipping 106 matching lines @@
     return orientation() == HorizontalScrollbar ? partLayoutObject->size().width() : partLayoutObject->size().height();
 }
 
 void LayoutScrollbar::invalidateDisplayItemClientsOfScrollbarParts(const LayoutBoxModelObject& paintInvalidationContainer, const LayoutRect& paintInvalidationRect)
 {
     for (auto& part : m_parts)
         part.value->invalidateDisplayItemClientsIncludingNonCompositingDescendants(&paintInvalidationContainer, PaintInvalidationScroll, &paintInvalidationRect);
 }
 
 }