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

third_party/WebKit/Source/core/paint/PaintInvalidationCapableScrollableArea.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef PaintInvalidationCapableScrollableArea_h
 #define PaintInvalidationCapableScrollableArea_h
 
 #include "core/CoreExport.h"
 #include "platform/geometry/LayoutRect.h"
 #include "platform/scroll/ScrollableArea.h"
 
 namespace blink {
 
 class LayoutBox;
 class LayoutBoxModelObject;
 class LayoutScrollbarPart;
 class PaintInvalidationContainer;
 class PaintInvalidationState;
 
 // Base class of FrameView and PaintLayerScrollableArea to share paint invalidation code.
 // TODO(wangxianzhu): Combine this into PaintLayerScrollableArea when root-layer-scrolls launches.
 class CORE_EXPORT PaintInvalidationCapableScrollableArea : public ScrollableArea {
 public:
     PaintInvalidationCapableScrollableArea()
         : m_horizontalScrollbarPreviouslyWasOverlay(false)
         , m_verticalScrollbarPreviouslyWasOverlay(false) { }
 
-    void willRemoveScrollbar(Scrollbar*, ScrollbarOrientation) override;
+    void willRemoveScrollbar(Scrollbar&, ScrollbarOrientation) override;
+
     void invalidatePaintOfScrollControlsIfNeeded(const PaintInvalidationState&, const LayoutBoxModelObject& paintInvalidationContainer);
 
     // Should be called when the previous paint invalidation rects are no longer valid.
     void clearPreviousPaintInvalidationRects();
 
 private:
     virtual LayoutBox& boxForScrollControlPaintInvalidation() const = 0;
     virtual LayoutScrollbarPart* scrollCorner() const = 0;
     virtual LayoutScrollbarPart* resizer() const = 0;
 
     bool m_horizontalScrollbarPreviouslyWasOverlay;
     bool m_verticalScrollbarPreviouslyWasOverlay;
     LayoutRect m_horizontalScrollbarPreviousPaintInvalidationRect;
     LayoutRect m_verticalScrollbarPreviousPaintInvalidationRect;
     LayoutRect m_scrollCornerPreviousPaintInvalidationRect;
 };
 
 } // namespace blink
 
 #endif // PaintInvalidationCapableScrollableArea_h