Network quality servicification of RTT and downlink estimates

Send the RTT and downlink estimates over the mojo channel
to the browser process. The RTT and downlink estimates are
sent over an existing mojo message which carrier the current
effective connection type estimate.

The mojo message is sent only if there is a significant change
in the values of either the RTT or the downlink estimate.

In the next CL, I will move the network quality consumers
in the browser process over to NetworkQualityTracker.

Cq-Include-Trybots: luci.chromium.try:linux_mojo
Change-Id: I01bbc7af2e7e3cf41f69e7ac19f8edd9ec59de90
Bug: 819244
Reviewed-on: https://chromium-review.googlesource.com/1130233
Commit-Queue: Tarun Bansal <tbansal@chromium.org>
Reviewed-by: Helen Li <xunjieli@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574685}
8 files changed