[New Multicolumn] Correct paint rectangle calculation for compositing.

Source/core/rendering/RenderLayer.cpp

Index: Source/core/rendering/RenderLayer.cpp
diff --git a/Source/core/rendering/RenderLayer.cpp b/Source/core/rendering/RenderLayer.cpp
index 9c306c9dd320254c78ad2ed96a6a0be03fe7c60c..de7234cdb0d11680e679e78f1793bed94151fa49 100644
--- a/Source/core/rendering/RenderLayer.cpp
+++ b/Source/core/rendering/RenderLayer.cpp
@@ -1491,7 +1491,14 @@ void RenderLayer::collectFragments(LayerFragments& fragments, const RenderLayer*
 
     // Shift the dirty rect into flow thread coordinates.
     LayoutPoint offsetOfPaginationLayerFromRoot;
-    enclosingPaginationLayer()->convertToLayerCoords(rootLayer, offsetOfPaginationLayerFromRoot);
+    if (rootLayer != enclosingPaginationLayer() && rootLayer->enclosingPaginationLayer() == enclosingPaginationLayer()) {
+        // The root layer is inside our pagination layer.
+        // FIXME: more work needed if there are nested pagination layers.
+        rootLayer->convertToLayerCoords(enclosingPaginationLayer(), offsetOfPaginationLayerFromRoot);
+        offsetOfPaginationLayerFromRoot = -offsetOfPaginationLayerFromRoot;
+    } else {
+        enclosingPaginationLayer()->convertToLayerCoords(rootLayer, offsetOfPaginationLayerFromRoot);
+    }
     LayoutRect dirtyRectInFlowThread(dirtyRect);
     dirtyRectInFlowThread.moveBy(-offsetOfPaginationLayerFromRoot);