Merged r12822 into 3.12 branch.

Set kChangesNewSpacePromotion for HStringAdd

R=danno@chromium.org

Review URL: https://chromiumcodereview.appspot.com/12217065

git-svn-id: https://v8.googlecode.com/svn/branches/3.12@13620 ce2b1a6d-e550-0410-aec6-3dcde31c8c00