[New Multicolumn] Simplified layout also needs to mark spanner placeholders for layout.

Source/core/layout/LayoutMultiColumnSpannerPlaceholder.h

Index: Source/core/layout/LayoutMultiColumnSpannerPlaceholder.h
diff --git a/Source/core/layout/LayoutMultiColumnSpannerPlaceholder.h b/Source/core/layout/LayoutMultiColumnSpannerPlaceholder.h
index 8f2b24d18cdd423cc227ab38eeb17f468ca8dcc7..1b48d4af8ff8ca97dbc1ceed513361b15f6aa8da 100644
--- a/Source/core/layout/LayoutMultiColumnSpannerPlaceholder.h
+++ b/Source/core/layout/LayoutMultiColumnSpannerPlaceholder.h
@@ -22,6 +22,14 @@ public:
     LayoutFlowThread* flowThread() const { return toLayoutBlockFlow(parent())->multiColumnFlowThread(); }
 
     LayoutBox* layoutObjectInFlowThread() const { return m_layoutObjectInFlowThread; }
+    void markForLayoutIfObjectInFlowThreadNeedsLayout()
+    {
+        if (!m_layoutObjectInFlowThread->needsLayout())
+            return;
+        // The containing block of a spanner is the multicol container (our parent here), but the
+        // spanner is laid out via its spanner set (us), so we need to make sure that we enter it.
+        setChildNeedsLayout(MarkOnlyThis);
+    }
     void updateMarginProperties();
 
     virtual const char* name() const override { return "LayoutMultiColumnSpannerPlaceholder"; }