Enforce a property whitelist on first-letter pseudo elements

This change prevents properties outside of a predefined whitelist [1] [2]
from being applied to first-letter pseudo elements.

In addition to the spec listed properties the following will continue to
be supported on first-letter pseudo elements:
 - -webkit-line-box-contain
 - visibility

[1] http://www.w3.org/TR/css3-selectors/#application-in-css
[2] http://www.w3.org/TR/css-text-decor-3/#text-shadow-property

BUG=360983, 361028

Review URL: https://codereview.chromium.org/263763004

git-svn-id: svn://svn.chromium.org/blink/trunk@173077 bbb929c8-8fbe-4397-9dbb-9b2b20218538