Make LayoutFlowThread::fragmentsBoundingBox() faster.

Source/core/layout/LayoutMultiColumnSet.cpp

Index: Source/core/layout/LayoutMultiColumnSet.cpp
diff --git a/Source/core/layout/LayoutMultiColumnSet.cpp b/Source/core/layout/LayoutMultiColumnSet.cpp
index 3882ab3f80e1d47d2d76c1a16c0c5e5de89521d0..e00b6f6114d76d3c802f8948f08ef156b30fea02 100644
--- a/Source/core/layout/LayoutMultiColumnSet.cpp
+++ b/Source/core/layout/LayoutMultiColumnSet.cpp
@@ -273,6 +273,14 @@ void LayoutMultiColumnSet::paintObject(const PaintInfo& paintInfo, const LayoutP
     MultiColumnSetPainter(*this).paintObject(paintInfo, paintOffset);
 }
 
+LayoutRect LayoutMultiColumnSet::fragmentsBoundingBox(const LayoutRect& boundingBoxInFlowThread) const
+{
+    LayoutRect result;
+    for (const auto& group : m_fragmentainerGroups)
+        result.unite(group.fragmentsBoundingBox(boundingBoxInFlowThread));
+    return result;
+}
+
 void LayoutMultiColumnSet::collectLayerFragments(DeprecatedPaintLayerFragments& fragments, const LayoutRect& layerBoundingBox, const LayoutRect& dirtyRect)
 {
     for (const auto& group : m_fragmentainerGroups)