Simplify zone discarding for preparsed functions by simply giving the preparser
its own zone that we reset whenever we finish preparsing something.

Change-Id: I3135fbbcd6caefa4654b1ae2b2207377e51bee26
Reviewed-on: https://chromium-review.googlesource.com/1238614
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56146}