Refactor out assumption that one byte strings are ascii in utf8 processing.

R=yangguo@chromium.org
BUG=

Review URL: https://chromiumcodereview.appspot.com/11725006
Patch from Dan Carney <dcarney@google.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13302 ce2b1a6d-e550-0410-aec6-3dcde31c8c00