LiftoffAssembler::ReserveStackSpace expected number of bytes, not
number of stack slots. Also, make this clear in the naming of the
parameter, and remove a dead field.

R=titzer@chromium.org

Bug: v8:6600
Change-Id: Ic4c7167cfa26a169b2936acdde865863141f441d
Reviewed-on: https://chromium-review.googlesource.com/823967Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50072}