Add logic to send connectivity probing packet to designated destination
using given packet writer. Probing packet effectively is a padded PING
packet.

relnote: n/a currently unused code.

Manual merge internal change: 175555522

Bug: 774622
Cq-Include-Trybots: master.tryserver.chromium.android:android_cronet_tester;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I3c355c31138fa7999c7a36e6b84cb548b701d91c
Reviewed-on: https://chromium-review.googlesource.com/767011
Reviewed-by: Ryan Hamilton <rch@chromium.org>
Commit-Queue: Zhongyi Shi <zhongyi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#516148}
11 files changed