Reporting: Fill in user_agent field in reports

Per https://github.com/w3c/reporting/issues/52, we are now supposed to
include the User-Agent string of the original request in any reports
generated about that request.

TBR=markusheintz@chromium.org

Bug: 865394
Cq-Include-Trybots: luci.chromium.try:linux_mojo
Change-Id: Ib49244399ed05398d598da1d02131153dd509306
Reviewed-on: https://chromium-review.googlesource.com/1140203
Commit-Queue: Douglas Creager <dcreager@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Reviewed-by: Misha Efimov <mef@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576599}
26 files changed