Unify paren handling in Parser and PreParser.

Unify paren handling in Parser and PreParser.

It is only needed in (Pre)Parser::ParseExpressionOrLabelledStatement for not
recognizing parenthesized identifiers as labels and in
(Pre)Parser::ParseSourceElements for not recognizing a parenthesized string as
directive prologue.

Parser Expressions don't keep track of whether they're parenthesized, so
PreParser Expressions shouldn't either.

In addition, add helper funcs for checking if an Expression is identifier or a
given identifier. (PreParser Expressions can do this.)

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

Review URL: https://codereview.chromium.org/150103004

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@19140 ce2b1a6d-e550-0410-aec6-3dcde31c8c00