Remove AtomicStringImpl.

This is attempt #2. Attempt #1 was reverted due to some object lifetime issues
which were tackled in separate bugs.

I also noted the paradigm of RefPtr<AtomicStringImpl> as a map key, which I
replaced with functionally-equivalent AtomicString. This may have the advantage
of less template specializations.

BUG=250050
TEST=layout tests under ASAN ok

Review URL: https://chromiumcodereview.appspot.com/19804005

git-svn-id: svn://svn.chromium.org/blink/trunk@154790 bbb929c8-8fbe-4397-9dbb-9b2b20218538