This CL fixes a crash when Structs are used as builtin arguments.

R=tebbi@chromium.org

Bug: v8:7793
Change-Id: Ib9488cdd924894dd9a9aa9fe35f57d93894565e2
Reviewed-on: https://chromium-review.googlesource.com/c/1447774Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Simon Zünd <szuend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59238}