No need to clear the style resolver in Document::childrenChanged

If you delete the documentElement then that also means all <style> and
<link> elements must have been removed from the document so the
StyleResolver should already be reset or cleared and we don't need to
call clearStyleResolver() from inside Document::childrenChanged.

Review URL: https://codereview.chromium.org/209383015

git-svn-id: svn://svn.chromium.org/blink/trunk@169911 bbb929c8-8fbe-4397-9dbb-9b2b20218538