Don’t merge: Fix CSSContentParser's use of 'Tidy'

This came up with failing framework unit tests locally:

```
1) CompositeFieldTest::testLegend
Failed asserting that two strings are equal.
--- Expected
+++ Actual
@@ @@
-'My legend'
+'My
+legend'
```

The -w option sets which column `tidy` should wrap at ([see man page](http://tidy.sourceforge.net/docs/tidy_man.html)). Without setting this, it’ll supposedly wrap at column 68 by default. The manual says that 0 is assumed if it’s omitted, but this doesn’t seem to always be the case (I’m on OS X 10.9.4 and that’s definitely not the case for me).

Marked as don’t merge as I wanted a second opinion before anyone clicks the green button - `CSSContentParser` isn’t exclusively used for unit tests, I’d hate to break something for someone (though I can’t see how/why someone would rely on wrapping at column 68!).