Network Error Logging: Add success and failure sampling rates

There are separate sampling rates for reports about successful and
failed requests.  Even though we're not currently reporting anything for
successful requests, this plumbs through both sampling rates from the
NEL header into the OriginPolicy instance.

(A future change will add reports for successful requests; doing the
patches in this order ensures that we never upload reports for 100% of
successful requests unless specifically instructed to do so by the
origin.)

Bug: 748549
Change-Id: I66d9d0c0544372e642d3a146deb0a06f7973452d
Reviewed-on: https://chromium-review.googlesource.com/891741
Commit-Queue: Julia Tuttle <juliatuttle@chromium.org>
Reviewed-by: Julia Tuttle <juliatuttle@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532911}
3 files changed