When peeking into descriptor arrays (for Function.prototype.bind
inlining), we need to check the number of descriptors rather than
the length of the DescriptorArray.

Bug: chromium:825045
Change-Id: I55dbe1544e5e4cb8e23d873961c71ed12294d89c
Reviewed-on: https://chromium-review.googlesource.com/991812Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52315}