Traitify ParserBase and move functions there.

The long-term goal is to move all recursive descent functions from Parser and
PreParser into ParserBase, but first they need to be unified.

Notes:
- The functions moved in this CL: ParseIdentifier, ParseIdentifierName,
ParseIdentifierNameOrGetOrSet, ParseIdentifierOrStrictReservedWord.
- IOW, this CL removes Parser::ParseIdentifier and PreParser::ParseIdentifier
  and adds ParserBase::ParseIdentifier, etc.
- Error reporting used to require virtual funcs; now error reporting is moved to
  the Traits too, and ParserBase no longer needs to be virtual.
- I had to move PreParser::Identifier out of the PreParser class, because
otherwise PreParserTraits cannot use it in a typedef.

BUG=v8:3126
LOG=N
R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/149913006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19230 ce2b1a6d-e550-0410-aec6-3dcde31c8c00