Only set up scroll bar layers if we have created the layers.

Source/core/page/scrolling/ScrollingCoordinator.cpp

Index: Source/core/page/scrolling/ScrollingCoordinator.cpp
diff --git a/Source/core/page/scrolling/ScrollingCoordinator.cpp b/Source/core/page/scrolling/ScrollingCoordinator.cpp
index 1f769dcf682eb453b9078b7ff02c5b003f2eeaac..20a515f1a02755f68f0e4d17f810ec3775f1f626 100644
--- a/Source/core/page/scrolling/ScrollingCoordinator.cpp
+++ b/Source/core/page/scrolling/ScrollingCoordinator.cpp
@@ -281,10 +281,16 @@ bool ScrollingCoordinator::scrollableAreaScrollLayerDidChange(ScrollableArea* sc
         webLayer->setScrollPosition(IntPoint(scrollableArea->scrollPosition() - scrollableArea->minimumScrollPosition()));
         webLayer->setMaxScrollPosition(IntSize(scrollableArea->scrollSize(HorizontalScrollbar), scrollableArea->scrollSize(VerticalScrollbar)));
     }
-    if (WebScrollbarLayer* scrollbarLayer = getWebScrollbarLayer(scrollableArea, HorizontalScrollbar))
-        setupScrollbarLayer(horizontalScrollbarLayerForScrollableArea(scrollableArea), scrollbarLayer, webLayer);
-    if (WebScrollbarLayer* scrollbarLayer = getWebScrollbarLayer(scrollableArea, VerticalScrollbar))
-        setupScrollbarLayer(verticalScrollbarLayerForScrollableArea(scrollableArea), scrollbarLayer, webLayer);
+    if (WebScrollbarLayer* scrollbarLayer = getWebScrollbarLayer(scrollableArea, HorizontalScrollbar)) {
+        GraphicsLayer* horizontalScrollbarLayer = horizontalScrollbarLayerForScrollableArea(scrollableArea);
+        if (horizontalScrollbarLayer)
+            setupScrollbarLayer(horizontalScrollbarLayer, scrollbarLayer, webLayer);
+    }
+    if (WebScrollbarLayer* scrollbarLayer = getWebScrollbarLayer(scrollableArea, VerticalScrollbar)) {
+        GraphicsLayer* verticalScrollbarLayer = verticalScrollbarLayerForScrollableArea(scrollableArea);
+        if (verticalScrollbarLayer)
+            setupScrollbarLayer(verticalScrollbarLayer, scrollbarLayer, webLayer);
+    }
 
     return !!webLayer;
 }