Prior to this, AllocateJSArray would go ahead and allocate an empty
FixedArray as elements if passed any capacity that is not a compile-time
constant 0.

Things break later on since we rely on the fact that empty fixed arrays
are always canonicalize, and we use

  obj.elements == empty_fixed_array_constant

interchangeably with

  obj.elements.length == 0.

This CL introduces two new branches in AllocateJSArray: one if the
capacity is known to be non-zero; and another that explicitly
distinguishes between 0 and non-zero capacities.

Bug: chromium:760790
Change-Id: I7c22b19ce9ce15a46f91b0f75e6b4a1ff3a29a0f
Reviewed-on: https://chromium-review.googlesource.com/645959
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47776}