Declare the private field variable in the preparser as well, to be
consistent with the parser.

Bug: v8:5386
Change-Id: I961ddf14e47b99701e2463cab0f4d4de140e1e3a
Reviewed-on: https://chromium-review.googlesource.com/905843Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Sathya Gunasekaran <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51154}