Reverse style resolution to avoid N^2 walk when building the render tree

Source/core/rendering/RenderRegion.cpp

Index: Source/core/rendering/RenderRegion.cpp
diff --git a/Source/core/rendering/RenderRegion.cpp b/Source/core/rendering/RenderRegion.cpp
index 651b8886e46fc280ce742a434250b37c6968da1b..680a98cd806b1dba8772a9e65a70649226a48631 100644
--- a/Source/core/rendering/RenderRegion.cpp
+++ b/Source/core/rendering/RenderRegion.cpp
@@ -550,7 +550,7 @@ PassRefPtr<RenderStyle> RenderRegion::computeStyleInRegion(const RenderObject* o
 
 void RenderRegion::computeChildrenStyleInRegion(const RenderObject* object)
 {
-    for (RenderObject* child = object->firstChild(); child; child = child->nextSibling()) {
+    for (RenderObject* child = object->lastChild(); child; child = child->previousSibling()) {
 
         RenderObjectRegionStyleMap::iterator it = m_renderObjectRegionStyle.find(child);
 
@@ -602,7 +602,7 @@ void RenderRegion::clearObjectStyleInRegion(const RenderObject* object)
     m_renderObjectRegionStyle.remove(object);
 
     // Clear the style for the children of this object.
-    for (RenderObject* child = object->firstChild(); child; child = child->nextSibling())
+    for (RenderObject* child = object->lastChild(); child; child = child->previousSibling())
         clearObjectStyleInRegion(child);
 }