Enable justified text for white-space: pre-wrap

According to the CSS3 spec, a UA can determine whether
or not it wishes to apply text-align: justify to text
with collapsible spaces[1]. Currently, we disable
whitespace expansion whenever we aren't collapsing
spaces, but this doesn't make a lot of sense for pre-
wrap text, since aside from not collapsing spaces,
flows text around lines like usual.

The patch to enable this is trivial, but should be
considered carefully. Currently no other browser
enables expansion for pre-wrap.

This was discussed on blink-dev[2] and www-style[3].

BUG=336636
[1]: http://www.w3.org/TR/css3-text/#text-align
[2]: https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/_Nnrn6FoVcE
[3]: http://lists.w3.org/Archives/Public/www-style/2014Apr/0469.html

Review URL: https://codereview.chromium.org/148013005

git-svn-id: svn://svn.chromium.org/blink/trunk@174895 bbb929c8-8fbe-4397-9dbb-9b2b20218538