AstNodeFactory used to get the Zone directly from AstValueFactory. But that's
generally the wrong Zone (the main Zone, instead of the temp Zone), and the
creator of AstNodeFactory had to call set_zone right after. By adding a Zone
param, we can pass the correct Zone right away.

Also made PreParserFactory have an AstNodeFactory, so that we don't need to
create temporary AstNodeFactories all the time.

Also removed AstNodeFactory::BodyScope since DiscardableZoneScope essentially
did the same thing already.

BUG=v8:5516,v8:6092

Change-Id: I189d2e6afe91c91e49d8ed7e3496a0d9c405e1c5
Reviewed-on: https://chromium-review.googlesource.com/507129
Commit-Queue: Marja Hölttä <marja@chromium.org>
Reviewed-by: Daniel Vogelheim <vogelheim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45370}