Revert of [Parse] ParseInfo owns the parsing Zone.

src/background-parsing-task.h

Index: src/background-parsing-task.h
diff --git a/src/background-parsing-task.h b/src/background-parsing-task.h
index 061e36595d1d388857059871a1425a9c4712b01a..d7fe6ba8db978b842e384c6ca762119f0c26f3fc 100644
--- a/src/background-parsing-task.h
+++ b/src/background-parsing-task.h
@@ -38,6 +38,7 @@
   // between parsing and compilation. These need to be initialized before the
   // compilation starts.
   UnicodeCache unicode_cache;
+  std::unique_ptr<Zone> zone;
   std::unique_ptr<ParseInfo> info;
   std::unique_ptr<Parser> parser;