Simplify the String management in CSSSelector::RareData

CSSSelector::setValue was manually managing the lifetime of a StringImpl*
from inside an AtomicString which was very confusing. Lets just use a real
String instead.

Review URL: https://codereview.chromium.org/93883007

git-svn-id: svn://svn.chromium.org/blink/trunk@164187 bbb929c8-8fbe-4397-9dbb-9b2b20218538