Get rid of PreParserScope.

Get rid of PreParserScope.

It's unnecessary; PreParser can just use normal Scopes for the things it needs
to track. Note: the only functionalities of PreParserScope were keeping track of
the scope stack, and for each scope, the scope type and language mode. Those are
now done by Scope. PreParser doesn't yet put variables into scopes (that will be
done in a follow up).

R=rossberg@chromium.org
BUG=

Review URL: https://codereview.chromium.org/912563004

Cr-Commit-Position: refs/heads/master@{#26544}