Fix base/numerics to not trigger upcoming Clang warning regarding
unsigned enum bitfields producing non-portable code.

For more details, see the patch here: https://reviews.llvm.org/D24289

BUG=648462

Change-Id: I9aaa01e99f734ff927b5b1171706b2f49b437d0b
Reviewed-on: https://chromium-review.googlesource.com/386887
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>