Unify PreParser::ParseIdentifierName and Parser::ParseIdentifierName.

No special handling for keywords is needed, since the literal ascii strings for
them work too (see how Parser did it).

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

Review URL: https://codereview.chromium.org/152853006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19184 ce2b1a6d-e550-0410-aec6-3dcde31c8c00