Implement handlified String::Flatten.

Implement handlified String::Flatten.

R=ulan@chromium.org

Committed: https://code.google.com/p/v8/source/detail?r=20572

[Yang, 2014-04-08 08:19:50]

This does not remove the use of the non-handlified String::TryFlatten yet. Some
possibly performance sensitive places use that and I'll deal with those in a
follow-up CL.

[Yang, 2014-04-08 09:33:25]

On 2014/04/08 08:19:50, Yang wrote:
> This does not remove the use of the non-handlified String::TryFlatten yet.
Some
> possibly performance sensitive places use that and I'll deal with those in a
> follow-up CL.

Changed as suggested.

[ulan, 2014-04-08 09:41:43]

lgtm

[Yang, 2014-04-08 09:50:02]

Committed patchset #2 manually as r20572 (presubmit successful).