Network Error Logging: Reports for successful requests and HTTP errors

We now generate NEL reports for successful requests, rate limited by the
`success-fraction` field of the NEL configuration header.  We have to
handle HTTP errors (i.e., those with 4xx and 5xx status codes)
specially, since they show up as "successful" according to the network
stack, but NEL considers that to be a special `http.error` failed
request.

Bug: 748549
Change-Id: I1a0a01fd52a4782855ba410be1301b0ccb81ee30
Reviewed-on: https://chromium-review.googlesource.com/893679
Commit-Queue: Douglas Creager <dcreager@google.com>
Reviewed-by: Julia Tuttle <juliatuttle@chromium.org>
Reviewed-by: Martin Šrámek <msramek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537526}
8 files changed