Let collapsed border drawings be cacheable

Source/core/layout/LayoutTableSection.cpp

 /*
  * Copyright (C) 1997 Martin Jones (mjones@kde.org)
  *           (C) 1997 Torben Weis (weis@kde.org)
  *           (C) 1998 Waldo Bastian (bastian@kde.org)
  *           (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  * Copyright (C) 2003, 2004, 2005, 2006, 2008, 2009, 2010, 2013 Apple Inc. All rights reserved.
  * Copyright (C) 2006 Alexey Proskuryakov (ap@nypop.com)
  *
  * This library is free software; you can redistribute it and/or
@@ skipping 1584 matching lines @@
 
 void LayoutTableSection::removeCachedCollapsedBorders(const LayoutTableCell* cell)
 {
     if (!table()->collapseBorders())
         return;
 
     for (int side = CBSBefore; side <= CBSEnd; ++side)
         m_cellsCollapsedBorders.remove(std::make_pair(cell, side));
 }
 
-void LayoutTableSection::setCachedCollapsedBorder(const LayoutTableCell* cell, CollapsedBorderSide side, const CollapsedBorderValue& border)
+bool LayoutTableSection::setCachedCollapsedBorder(const LayoutTableCell* cell, CollapsedBorderSide side, const CollapsedBorderValue& border)
 {
     ASSERT(table()->collapseBorders());
     CellsCollapsedBordersMap::iterator it = m_cellsCollapsedBorders.find(std::make_pair(cell, side));
-    if (it == m_cellsCollapsedBorders.end())
+    if (it == m_cellsCollapsedBorders.end()) {
         m_cellsCollapsedBorders.add(std::make_pair(cell, side), border);
-    else
+        return true;
+    }
+    if (!it->value.equals(border)) {
         it->value = border;
+        return true;
+    }
+    return false;
 }
 
 const CollapsedBorderValue& LayoutTableSection::cachedCollapsedBorder(const LayoutTableCell* cell, CollapsedBorderSide side) const
 {
     ASSERT(table()->collapseBorders());
     CellsCollapsedBordersMap::const_iterator it = m_cellsCollapsedBorders.find(std::make_pair(cell, side));
     ASSERT_WITH_SECURITY_IMPLICATION(it != m_cellsCollapsedBorders.end());
     return it->value;
 }
 
@@ skipping 14 matching lines @@
     // FIXME: The table's direction should determine our row's direction, not the section's (see bug 96691).
     if (!style()->isLeftToRightDirection())
         cellLocation.setX(table()->columnPositions()[table()->numEffCols()] - table()->columnPositions()[table()->colToEffCol(cell->col() + cell->colSpan())] + horizontalBorderSpacing);
     else
         cellLocation.setX(table()->columnPositions()[effectiveColumn] + horizontalBorderSpacing);
 
     cell->setLogicalLocation(cellLocation);
 }
 
 } // namespace blink