We are only calling {checkAvailable} with uint32_t, thus the type
received should also be uint32_t. This also gives better error messages
for integers >kMaxInt.

R=titzer@chromium.org

Change-Id: I40655b80dcc601e05fbb0c92cedba72ca806e2b3
Reviewed-on: https://chromium-review.googlesource.com/605893
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47227}