Few more details:
1. NQE classes in metrics service are initialized on IO thread. Metrics service initialization happens on UI thread.
2. When NQE code was written, merics service initialization happened after IO thread initialization. So, metrics service would simply post a task to IO thread for getting NQE initialized.
3. A recent CL (https://chromium-review.googlesource.com/c/586572) broke that assumption which means the post task to IO thread always failed resulting in failure of NQE initialization.

According to the documentation (https://chromium.googlesource.com/chromium/src/+/lkcr/docs/task_scheduler_migration.md): "All BrowserThreads but UI/IO are being migrated ..."
So, it seems that unlike other threads, UI/IO threads have to be initialized before they accept post tasks?

> According to the documentation (https://chromium.googlesource.com/chromium/src/+/lkcr/docs/task_scheduler_migration.md): "All BrowserThreads but UI/IO are being migrated ..."
So, it seems that unlike other threads, UI/IO threads have to be initialized before they accept post tasks?

Ugh... yeah... that's always been the case FWIW, BrowserThreads have never accepted tasks prior to CreateThreads(). Now that TaskScheduler does support posting before CreateThreads (results in queuing) and the only BrowserThreads will soon be just UI/IO we could consider queuing for these as well.

But you're right that https://chromium-review.googlesource.com/c/586572 broke MetricsService's assumption that the IO thread was up during its initialization...

(more replies on CL)

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/7e299f59fcbd38ba654c71b742e4c3616dda7463

commit 7e299f59fcbd38ba654c71b742e4c3616dda7463
Author: Tarun Bansal <tbansal@chromium.org>
Date: Wed Aug 16 20:12:28 2017

Fix recording of Effective connection type (ECT) in system profile proto

ECT may not be recorded in certain cases where metrics service gets
initialized before the IO thread is initialized. In that case, posting
tasks to the IO thread fails.

This CL fixes the issue by waiting for the IO thread to be initialized
before posting tasks to it.

Bug:  755314 
Change-Id: I37e1c1362bb76e8d2b47ec7bb408d0282287d532
Reviewed-on: https://chromium-review.googlesource.com/614775
Reviewed-by: Alexei Svitkine (very slow) <asvitkine@chromium.org>
Commit-Queue: Tarun Bansal <tbansal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#494917}
[modify] https://crrev.com/7e299f59fcbd38ba654c71b742e4c3616dda7463/chrome/browser/metrics/network_quality_estimator_provider_impl.cc
[modify] https://crrev.com/7e299f59fcbd38ba654c71b742e4c3616dda7463/chrome/browser/metrics/network_quality_estimator_provider_impl.h
[modify] https://crrev.com/7e299f59fcbd38ba654c71b742e4c3616dda7463/components/metrics/net/network_metrics_provider.cc
[modify] https://crrev.com/7e299f59fcbd38ba654c71b742e4c3616dda7463/components/metrics/net/network_metrics_provider.h
[modify] https://crrev.com/7e299f59fcbd38ba654c71b742e4c3616dda7463/components/metrics/net/network_metrics_provider_unittest.cc