This fixes an issue introduced in https://crrev.com/c/1301483.

The JSArray allocation could trigger GC and thus elements must be
fully initialized.

Bug: v8:8429,chromium:890599
Change-Id: I7bfa1728c1dde7fc880063e095413163b13be2d5
Reviewed-on: https://chromium-review.googlesource.com/c/1322955Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57342}