[New Multicolumn] Handle flow thread overflow correctly in flipped writing modes.

Source/core/layout/MultiColumnFragmentainerGroup.cpp

Index: Source/core/layout/MultiColumnFragmentainerGroup.cpp
diff --git a/Source/core/layout/MultiColumnFragmentainerGroup.cpp b/Source/core/layout/MultiColumnFragmentainerGroup.cpp
index 733ac93aeefcbe72475162f4234da25a289f841a..1b3e317cbe681092ab12cab97850b10a1def8d7f 100644
--- a/Source/core/layout/MultiColumnFragmentainerGroup.cpp
+++ b/Source/core/layout/MultiColumnFragmentainerGroup.cpp
@@ -157,7 +157,8 @@ void MultiColumnFragmentainerGroup::collectLayerFragments(LayerFragments& fragme
     // Then there's the output from this method - the stuff we put into the list of fragments. The
     // fragment.paginationOffset point is the actual visual translation required to get from a
     // location in the flow thread to a location in a given column. The fragment.paginationClip
-    // rectangle, on the other hand, is in flow thread coordinates.
+    // rectangle, on the other hand, is in flow thread coordinates, but otherwise completely
+    // physical in terms of writing mode.
     //
     // All other rectangles in this method are sized physically, and the inline direction coordinate
     // is physical too, but the block direction coordinate is "logical top". This is the same as