This CL removes a CHECK_LE that does not hold in all cases. After
moving all elements to the front, current_pos will point to the next
free spot. In the case where an object is 'packed', i.e. each index
has a non-undefined value, and the length is smaller then the max
index, current_pos will be greater than the length (limit in the code).

Sidenote: The block after taking the minimum (where the counted
undefineds get set) will not be affected. In the case where
num_undefined > 0, current_pos should be guaranteed to be smaller
than limit, as long there are no accessors with side-effects.

R=jgruber@chromium.org

Bug: chromium:923265
Change-Id: Id533cdc4db6c6c6f266cf7c6a8ab6ecbbeee7016
Reviewed-on: https://chromium-review.googlesource.com/c/1420679Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Simon Zünd <szuend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58912}