Introduce 'contextual keyword' tokens, which are parsed as identifiers but
in some contexts are treated by the parser like proper keywords. These are
usually keywords introduced by recent ECMAScript versions, which for reasons
of backwards compatibility are still permissible as regular identifiers in
most contexts.

Current usage is to check for Token::IDENTIFIER and then do a string
compare. With this change the initial scan will scan them as usual, but
will then record the token as IDENTIFIER plus a secondary token with the
'contextual' value.

BUG=v8:6902

Change-Id: I6ae390382998cf756a23720bd481cb9c0eb78a72
Reviewed-on: https://chromium-review.googlesource.com/459479
Commit-Queue: Daniel Vogelheim <vogelheim@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44189}