The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/01c4f97005f6f162b3df6953b8f2a4efce338ec1

commit 01c4f97005f6f162b3df6953b8f2a4efce338ec1
Author: Tarun Bansal <tbansal@chromium.org>
Date: Fri Dec 07 06:11:36 2018

Report PreviewsEligibilityReason as offline when device is offline

Currently, if the device is offline, PreviewsEligibilityReason
records the reason for not showing previews as network quality
unavailable.

This CL changes it to record the reason as device offline.
Network quality unavailable reason is now used for only the cases where
NQE (network quality estimator) reports ECT as Unknown.

Bug:  912725 
Change-Id: I0af0977770e2123f6ccd0317f8ebbfaed1aac9b2
Reviewed-on: https://chromium-review.googlesource.com/c/1366835
Commit-Queue: Tarun Bansal <tbansal@chromium.org>
Reviewed-by: Ryan Sturm <ryansturm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#614621}
[modify] https://crrev.com/01c4f97005f6f162b3df6953b8f2a4efce338ec1/components/previews/content/previews_decider_impl.cc
[modify] https://crrev.com/01c4f97005f6f162b3df6953b8f2a4efce338ec1/components/previews/content/previews_decider_impl_unittest.cc
[modify] https://crrev.com/01c4f97005f6f162b3df6953b8f2a4efce338ec1/components/previews/core/previews_black_list.h
[modify] https://crrev.com/01c4f97005f6f162b3df6953b8f2a4efce338ec1/components/previews/core/previews_logger.cc
[modify] https://crrev.com/01c4f97005f6f162b3df6953b8f2a4efce338ec1/tools/metrics/histograms/enums.xml