Remove AtomicStringImpl.

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

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=154790

[Chris Evans, 2013-07-23 20:30:57]

This should land and stick this time.

[abarth-chromium, 2013-07-23 20:33:12]

lgtm

[commit-bot: I haz the power, 2013-07-23 21:03:19]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/cevans@chromium.org/19804005/1

[commit-bot: I haz the power, 2013-07-23 23:04:34]

Change committed as 154790