Revert "Do not specify font-stretch in @font-face selectors."

This reverts commit 9d064e7f7d3f0b4e0ea9fb3c8c851a076b10dc6e.
CSS @font-face declarations now retain existing symbolicTraits.

In iOS 8, some fonts like "Avenir Next Condensed" have additional symbolic traits set (in this case, UIFontDescriptorTraitCondensed). This change ensures that they are preserved while modifying only the italic and bold traits. Otherwise with the trait removed they may not match any font.