Clean up Document::recalcStyle

We can add some early returns and avoid null checks and the goto inside Document::recalcStyle.

Review URL: https://chromiumcodereview.appspot.com/23532039

git-svn-id: svn://svn.chromium.org/blink/trunk@158415 bbb929c8-8fbe-4397-9dbb-9b2b20218538