Avoid stack overflow triggered by out-of-band layout of flexbox child.

Source/core/layout/LayoutMultiColumnFlowThread.h

Index: Source/core/layout/LayoutMultiColumnFlowThread.h
diff --git a/Source/core/layout/LayoutMultiColumnFlowThread.h b/Source/core/layout/LayoutMultiColumnFlowThread.h
index a67c9c91e96ef4b67020f3cdb32544057bd906b3..3f36cef12c285dd5ae5bf168459532b1e815d717 100644
--- a/Source/core/layout/LayoutMultiColumnFlowThread.h
+++ b/Source/core/layout/LayoutMultiColumnFlowThread.h
@@ -180,6 +180,11 @@ public:
 
     bool isInInitialLayoutPass() const { return !m_inBalancingPass; }
 
+    // Skip past a column spanner during flow thread layout. Spanners are not laid out inside the
+    // flow thread, since the flow thread is not in a spanner's containing block chain (since the
+    // containing block is the multicol container).
+    void skipColumnSpanner(LayoutBox*, LayoutUnit logicalTopInFlowThread);
+
     bool recalculateColumnHeights();
 
     void columnRuleStyleDidChange();
@@ -215,7 +220,6 @@ private:
 
     void addColumnSetToThread(LayoutMultiColumnSet*) override;
     void willBeRemovedFromTree() override;
-    void skipColumnSpanner(LayoutBox*, LayoutUnit logicalTopInFlowThread) override;
     void flowThreadDescendantWasInserted(LayoutObject*) final;
     void flowThreadDescendantWillBeRemoved(LayoutObject*) final;
     void flowThreadDescendantStyleWillChange(LayoutObject*, StyleDifference, const ComputedStyle& newStyle) override;