Ensure that positioned objects' list is always in parent first order

Source/core/layout/LayoutBlock.cpp

Index: Source/core/layout/LayoutBlock.cpp
diff --git a/Source/core/layout/LayoutBlock.cpp b/Source/core/layout/LayoutBlock.cpp
index dd9facbf773689c89c04a2fa25bf0158e740a832..10eb36fe5e9cce8a0d519d85c7da9a1b63a9c2b3 100644
--- a/Source/core/layout/LayoutBlock.cpp
+++ b/Source/core/layout/LayoutBlock.cpp
@@ -281,20 +281,11 @@ void LayoutBlock::styleWillChange(StyleDifference diff, const ComputedStyle& new
             // Clear our positioned objects list. Our absolutely positioned descendants will be
             // inserted into our containing block's positioned objects list during layout.
             removePositionedObjects(0, NewContainingBlock);
-        } else if (!oldStyleIsContainer && newStyleIsContainer) {
+        } else if ((!oldStyleIsContainer || oldStyle->position() != newStyle.position()) && newStyleIsContainer) {
             // Remove our absolutely positioned descendants from their current containing block.

[mstensho (USE GERRIT), 2015/06/15 10:02:55]

This comment is now inaccurate. We're removing positioned descendants from the
*new* containing block.

             // They will be inserted into our positioned objects list during layout.
-            LayoutObject* cb = parent();
-            while (cb && (cb->style()->position() == StaticPosition || (cb->isInline() && !cb->isReplaced())) && !cb->isLayoutView()) {
-                if (cb->style()->position() == RelativePosition && cb->isInline() && !cb->isReplaced()) {
-                    cb = cb->containingBlock();
-                    break;
-                }
-                cb = cb->parent();
-            }
-
-            if (cb->isLayoutBlock())
-                toLayoutBlock(cb)->removePositionedObjects(this, NewContainingBlock);
+            if (LayoutBlock* cb = containingBlock(&newStyle))

[mstensho (USE GERRIT), 2015/06/15 10:02:54]

Why not do this in styleDidChange() instead of styleWillChange? Then we wouldn't
need the optional overrideStyle parameter to containingBlock().

The entire code block with the oldStyleIsContainer and newStyleIsContainer stuff
could be moved there.

+                cb->removePositionedObjects(this, NewContainingBlock);
         }
     }