This is just a rename. After commit, I'll begin with the semantic changes.
Until those are complete, kOneByteStringTag will have the same meaning as
kAsciiStringTag.

BUG=

Review URL: https://chromiumcodereview.appspot.com/11293168
Patch from Dan Carney <dcarney@google.com>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12897 ce2b1a6d-e550-0410-aec6-3dcde31c8c00