Consistently use HStringAdd instead of HCallStub with CodeStub::StringAdd.

Consistently use HStringAdd instead of HCallStub with CodeStub::StringAdd.

Previously there were two ways to actually use the StringAddStub
from Hydrogen:

- Either using HStringAdd (which implied NO_STRING_CHECK_IN_STUB
  and and does the argument handling internally),
- or using HCallStub with CodeStub::StringAdd (which implied
  NO_STRING_ADD_FLAGS and expected the arguments to be on the
  stack already).

R=mvstanton@chromium.org

Review URL: https://codereview.chromium.org/19541003

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@15771 ce2b1a6d-e550-0410-aec6-3dcde31c8c00