Add mechanism to add RTT/throughput estimates observer to NQT

Add mechanism to add HTTP RTT, transport RTT, downlink throughout
estimate change observer to NetworkQualityTracker.

In the next CL, an IO object owned by UINetworkQualityEstimatorService
would subscribe to these changes, and then pass them on to the UI
thread.

Bug: 819244
Cq-Include-Trybots: luci.chromium.try:linux_mojo
Change-Id: Ieeb786c9376af7afe9f2e8bff85468067c4c4ecf
Reviewed-on: https://chromium-review.googlesource.com/1144470
Reviewed-by: Nick Carter <nick@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Commit-Queue: Tarun Bansal <tbansal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580367}
4 files changed