ContainerNode::removeChildren should first detach the children then remove them
https://bugs.webkit.org/show_bug.cgi?id=113433

Reviewed by Ryosuke Niwa.

Currently, ContainerNode::removeChildren initially removes the nodes from the DOM tree and then
calls detach() on each of them. This is anti-intuitive and can lead to subtle bugs because the
detached renderers are not backed by a valid DOM tree any more. With the patch, the nodes are first
detached and then removed from the DOM.
The patch also lets the tree in a consistent state after each node removal by clearing the previous
sibling pointer of the node following the one removed.
I haven't found any proof the performance will get worse if the detachment happens when the children
are still in the DOM tree.

Tests: No changed visible functionality.

* dom/ContainerNode.cpp:
(WebCore::ContainerNode::removeChildren):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@148754 268f45cc-cd09-0410-ab3c-d52691b4dbfc