For stack sizes and control depths, we were sometimes using uint32_t
and sometimes size_t.
This CL switches to uint32_t consistently.

R=titzer@chromium.org

Change-Id: I5ce3d63832bc926584b153cf248006cd78d77b97
Reviewed-on: https://chromium-review.googlesource.com/645861Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47769}