Port r10070 (4d7e5a8a)

Original commit message:
TEST=test/mjsunit/string-external-cached.js

BUG=

Review URL: http://codereview.chromium.org/8744012
Patch from Daniel Kalmar <kalmard@homejinni.com>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@10109 ce2b1a6d-e550-0410-aec6-3dcde31c8c00