Add and populate subdirectory OWNERS files in net/.

Keeping the top-level OWNERS as-is, for now.

Bug: none
Cq-Include-Trybots: master.tryserver.chromium.android:android_cronet_tester;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I7a1f846ff6710fa5e15275b9cbc1b4637d74a5b0
Reviewed-on: https://chromium-review.googlesource.com/744941
Commit-Queue: Matt Menke <mmenke@chromium.org>
Reviewed-by: Misha Efimov <mef@chromium.org>
Reviewed-by: Ryan Sleevi <rsleevi@chromium.org>
Reviewed-by: Eric Roman <eroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#512973}

• net/cert/OWNERS[diff]
• net/cert_net/OWNERS[diff]
• net/cookies/OWNERS[diff]
• net/der/OWNERS[Added - diff]
• net/disk_cache/OWNERS[diff]
• net/dns/OWNERS[diff]
• net/filter/OWNERS[diff]
• net/ftp/OWNERS[diff]
• net/http2/OWNERS[diff]
• net/log/OWNERS[diff]
• net/proxy/OWNERS[diff]
• net/quic/OWNERS[diff]
• net/reporting/OWNERS[Added - diff]
• net/spdy/OWNERS[diff]
• net/ssl/OWNERS[diff]