Network quality servicification of transport RTT

Send the transport RTT over the mojo channel to the
browser process. The transport RTT is sent over an
existing mojo message which carrier the other network
quality estimates.

The mojo message is sent only if there is a significant change
in the values of the estimates.

Cq-Include-Trybots: luci.chromium.try:linux_mojo
Change-Id: Ief33a27239e88b368cf403b4578bfde329bbabbf
Bug: 819244
Reviewed-on: https://chromium-review.googlesource.com/1139084
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Helen Li <xunjieli@chromium.org>
Commit-Queue: Tarun Bansal <tbansal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575636}
8 files changed