This prevents us from accidentally overwriting the default map of the
empty_fixed_array since NewFixedArray might just return that.

Change-Id: Id2fd20db1c07ec0289905ce5b0e12b5530a60229
Reviewed-on: https://chromium-review.googlesource.com/942923Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51692}