simplify even more the new setlength impl

Bug: skia:
Change-Id: I160de9807369125d49774deefd1402c5a39ae41f
Reviewed-on: https://skia-review.googlesource.com/c/193360
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Mike Reed <reed@google.com>
1 file changed