The style guide says that only `int` should be used of the builtin
integer types. Instead, we should use the stdint types.

See https://google.github.io/styleguide/cppguide.html#Integer_Types

Change-Id: I1af53a3bceefbfed85589b74a602c8ebe1c7ee25
Reviewed-on: https://chromium-review.googlesource.com/c/1342663Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Ben Smith <binji@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57851}