Fix assertion failure caused by external strings.

Fix assertion failure caused by external strings.

This fixes two issues:
- Update externalize-string-extension to the behavior of the API (see r18285)
- Convert cons strings in old pointer space to short external strings as
  expected by Heap::AllowedToBeMigrated, regardless of alignment.

R=mstarzinger@chromium.org

Committed: https://code.google.com/p/v8/source/detail?r=19755

[Yang, 2014-03-10 11:42:05]

This fixes some GC stress failures.

[Michael Starzinger, 2014-03-10 11:47:14]

LGTM.

[Yang, 2014-03-10 12:08:24]

Committed patchset #1 manually as r19755 (presubmit successful).