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Add histogram to record how frequently network quality
is detected as meaningfully changed.

Also, add DCHECKs to verify that the RTTs and throughput
values are as unexpected.

Cq-Include-Trybots: master.tryserver.chromium.android:android_cronet_tester;master.tryserver.chromium.mac:ios-simulator-cronet
Bug: 798488
Change-Id: Ie0bbf1f691a002a9257bff4c089e9dc8d0156b86
Reviewed-on: https://chromium-review.googlesource.com/846285Reviewed-by: Helen Li <xunjieli@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Doug Arnett <dougarnett@chromium.org>
Commit-Queue: Tarun Bansal <tbansal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#526898}