NEL: Create Delegate interface in //net/url_request, plumb headers out.

Bug: 748549
Change-Id: Ib622dda648649bbcd9d0a2f16307dc700223c068
Reviewed-on: https://chromium-review.googlesource.com/580268
Commit-Queue: Julia Tuttle <juliatuttle@chromium.org>
Reviewed-by: Miriam Gershenson <mgersh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#489921}
10 files changed