Fix crash on executing URL with hashtag(local bookmark)
Refactor self.recomposite value update
auto merge of #1447 : june0cho/servo/fix_crash_on_local_bookmark, r=kmcallister

Local bookmark which is implemented in #1262 is not currently working when executing URL with hashtag(e.g. `test_local_bookmark.html#h3`, `acid2.html#top`).
Local bookmark has two matching processes(http://www.w3.org/TR/html5/browsers.html#scroll-to-fragid). Among two, this commit fixes the crash on ID attribute, and still has crash on `a` element with `name` attribute.
As a result, with this commit, `test_local_bookmark.html#h3` works well, but `acid2.html#top` still does not work.