Simplify CSSParserToken::textForUnitTests by using more WTF helpers

Instead of manually handling char arrays and using sprintf in converting
CSSParserTokens to strings, we should just concatenate Strings and use
String::number. I didn't bother using StringBuilder here since the
function is only used in unit tests and it's a bit more readable this way.

Review URL: https://codereview.chromium.org/667663002

git-svn-id: svn://svn.chromium.org/blink/trunk@183975 bbb929c8-8fbe-4397-9dbb-9b2b20218538