metrics: Use unsigned type for length field

This brings the changes from the Chrome OS CL
https://chromium-review.googlesource.com/c/chromiumos/platform2/+/1341794
over to the Chrome repository.

BUG=chromium:906622
TEST=Compiles and passes tests.

Change-Id: I36b4c6d156322ca56d8678b10ac7b46103e3c670
Reviewed-on: https://chromium-review.googlesource.com/c/1346397
Commit-Queue: Mattias Nissler <mnissler@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#619913}
3 files changed