Clear scrollbar damages in time

Source/core/rendering/RenderLayerScrollableArea.cpp

Index: Source/core/rendering/RenderLayerScrollableArea.cpp
diff --git a/Source/core/rendering/RenderLayerScrollableArea.cpp b/Source/core/rendering/RenderLayerScrollableArea.cpp
index 052e0aa3c0c2db3ff6961f0c9a125a5eec6405b8..42bd5b19e90561ebb6c795e207cad8e84f5ff125 100644
--- a/Source/core/rendering/RenderLayerScrollableArea.cpp
+++ b/Source/core/rendering/RenderLayerScrollableArea.cpp
@@ -207,13 +207,13 @@ void RenderLayerScrollableArea::invalidateScrollbarRect(Scrollbar* scrollbar, co
 
     if (box().frameView()->isInPerformLayout()) {
         if (scrollbar == m_vBar.get()) {
-            m_verticalBarDamage = intRect;
+            m_verticalBarDamage.unite(intRect);
             m_hasVerticalBarDamage = true;
         } else {
-            m_horizontalBarDamage = intRect;
+            m_horizontalBarDamage.unite(intRect);
             m_hasHorizontalBarDamage = true;
         }
-
+        box().setMayNeedPaintInvalidation(true);
     } else {
         box().invalidatePaintRectangle(intRect);
     }