Unify function name validation in Parser and PreParser.

BUG=3126
LOG=N
R=ulan@chromium.org

Review URL: https://codereview.chromium.org/140183005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19193 ce2b1a6d-e550-0410-aec6-3dcde31c8c00