Update from latest upstream webkit

commit 2336db4154ba0c8dc3848068f6b95ea185aa43e2
Author: akling@apple.com <akling@apple.com@268f45cc-cd09-0410-ab3c-d52691b4dbfc>
Date:   Mon Sep 23 13:04:43 2013 +0000

    Unify code paths leading to render tree teardown.
    <https://webkit.org/b/121785>

    Reviewed by Antti Koivisto.

    Make Document::prepareForDestruction() fit all the call sites that were
    doing destroyRenderTree() + disconnectFromFrame().

    - disconnectDescendantFrames() is already a no-op if there are no frames.
    - willDetachDocumentFromFrame() will only run if there is a frame.

    This is another step towards making tree rebuild possible.

    Removed some age-old FIXMEs about why we aren't calling willRemove(),
    a long-gone function.

    git-svn-id: http://svn.webkit.org/repository/webkit/trunk@156270 268f45cc-cd09-0410-ab3c-d52691b4dbfc