Fix under-invalidation of frame scrollbar on hover

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"
@@ skipping 19 matching lines @@
     void invalidatePaintOfScrollControlsIfNeeded(const PaintInvalidationState&);
 
     // Should be called when the previous paint invalidation rects are no longer valid.
     void clearPreviousPaintInvalidationRects();
 
     virtual IntRect scrollCornerAndResizerRect() const { return scrollCornerRect(); }
 
     LayoutRect visualRectForScrollbarParts() const override;
 
 private:
-    virtual LayoutBox& boxForScrollControlPaintInvalidation() const = 0;
     virtual LayoutScrollbarPart* scrollCorner() const = 0;
     virtual LayoutScrollbarPart* resizer() const = 0;
 
+    void scrollControlWasSetNeedsPaintInvalidation() override;
+
     bool m_horizontalScrollbarPreviouslyWasOverlay;
     bool m_verticalScrollbarPreviouslyWasOverlay;
     LayoutRect m_horizontalScrollbarPreviousPaintInvalidationRect;
     LayoutRect m_verticalScrollbarPreviousPaintInvalidationRect;
     LayoutRect m_scrollCornerAndResizerPreviousPaintInvalidationRect;
 };
 
 } // namespace blink
 
 #endif // PaintInvalidationCapableScrollableArea_h