Don't set document.title when there is no <head>

Align setting document.title with

http://www.whatwg.org/specs/web-apps/current-work/multipage/dom.html#document.title

a bit closer by refusing to update the document title when there is no
head element.

BUG=330165

Review URL: https://codereview.chromium.org/98373012

git-svn-id: svn://svn.chromium.org/blink/trunk@164582 bbb929c8-8fbe-4397-9dbb-9b2b20218538