Restrict <abbr> formatting to where a title attribute is present

Reasoning:

* It does not make sense to draw attention to an <abbr> element that
  doesn't have a title attribute because there is nothing special to
  see there.
* It matches Gecko's default stylesheet [1] and the HTML5 style [2].
* It does not negatively impact compatibility, as far as I know
  (IE 6 does not support the [title] selector, but does not support
  the <abbr> element either).
* <acronym> elements are currently not allowed and won't be because
  they are non-conforming in HTML5 (cf. bug 671, comment 37), so this
  part of the rule can be dropped.

[1] http://mxr.mozilla.org/mozilla-central/source/layout/style/html.css
[2] http://www.whatwg.org/specs/web-apps/current-work/multipage/rendering.html#phrasing-content-1

Bug: 54729
Change-Id: I1b00ddb7996911e9b477365e7a84458cefd5da90