Move CSS RuleFeature away from AtomicStringImpl.

We now use AtomicString directly instead, to avoid an object lifetime issue.

BUG=257145
TEST=LayoutTests under ASAN

Review URL: https://chromiumcodereview.appspot.com/19991003

git-svn-id: svn://svn.chromium.org/blink/trunk@154724 bbb929c8-8fbe-4397-9dbb-9b2b20218538