Make LayoutFlowThread::fragmentsBoundingBox() faster.

Source/core/layout/LayoutMultiColumnSet.cpp

 /*
  * Copyright (C) 2012 Apple Inc.  All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 255 matching lines @@
     // FIXME: remove this method. It's a meaningless question to ask the set "how many columns do
     // you actually have?", since that may vary for each row.
     return firstFragmentainerGroup().actualColumnCount();
 }
 
 void LayoutMultiColumnSet::paintObject(const PaintInfo& paintInfo, const LayoutPoint& paintOffset)
 {
     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)
         group.collectLayerFragments(fragments, layerBoundingBox, dirtyRect);
 }
 
 void LayoutMultiColumnSet::addOverflowFromChildren()
 {
     LayoutRect overflowRect;
     for (const auto& group : m_fragmentainerGroups) {
@@ skipping 39 matching lines @@
 
 LayoutRect LayoutMultiColumnSet::flowThreadPortionRect() const
 {
     LayoutRect portionRect(LayoutUnit(), logicalTopInFlowThread(), pageLogicalWidth(), logicalHeightInFlowThread());
     if (!isHorizontalWritingMode())
         return portionRect.transposedRect();
     return portionRect;
 }
 
 }