Don't create an empty text node when setting document.title to the empty string

http://www.whatwg.org/specs/web-apps/current-work/#document.title
http://www.whatwg.org/specs/web-apps/current-work/#the-title-element

now both refer to:

http://dom.spec.whatwg.org/#dom-node-textcontent

for how the empty string should handled: empty string -> no child
element. Note that the act of updating the document's copy of the
title then falls on the removeChildren() call.

Since FF and IE10 already follows the spec, web compatibility risk
should be small.

BUG=336154

Review URL: https://codereview.chromium.org/143363002

git-svn-id: svn://svn.chromium.org/blink/trunk@165453 bbb929c8-8fbe-4397-9dbb-9b2b20218538