Network Error Logging: Update payload fields to use underscores

This updates the NEL implementation to match the fields renames from
https://github.com/WICG/network-error-logging/pull/75.

Bug: 748549
Cq-Include-Trybots: luci.chromium.try:closure_compilation
Change-Id: I0545ce6f9bddb46c556e2ba15d16398ee9c3b1fa
Reviewed-on: https://chromium-review.googlesource.com/1104384
Reviewed-by: Matt Menke <mmenke@chromium.org>
Commit-Queue: Douglas Creager <dcreager@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568048}
4 files changed