Always parse through ParseFunctionBody to avoid bugs with parameter/scope
handling.

Change-Id: Ia0e78c6b3127e99f92a6c772ba2be509f6379f5a
Reviewed-on: https://chromium-review.googlesource.com/c/1268236
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56445}