For spread calls with arrays with double elements but zero length,
we skip the box-as-heapnumber step; so in this corner case the
Call builtin sees a FixedDoubleArray, which is fine because it
doesn't read any of the raw double values from it.
This patch doesn't change the implementation, it only updates the
assert to match reality.

Bug: chromium:856095
Change-Id: I0227f4ccbc6c61c8f5f7669a266ef7a64c6a9a43
Reviewed-on: https://chromium-review.googlesource.com/1117922Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54149}