This issue comes from https://codereview.chromium.org/2402983002/, which introduces a blink platform API isMessageLoopReady() to help check whether there is a message loop ready, if not, blink::initialize() will skip some mojo stuff(only TimeZoneMonitorClient for now) initialization.

While production code is always getting message loop ready before calling blink::initialize(), some unit tests code are not, this means the mojo stuff(only TimeZoneMonitorClient for now) initialization will be skipped silently for those unit tests:
unit_tests, content_unittests, components_unittests, extensions_unittests,
webkit_unit_tests and blink_fuzzer_test_support

> A bit more context:
> 
> -
> https://groups.google.com/a/chromium.org/d/topic/chromium-mojo/2Z3W2aQqBZY/discussion
> -
> https://cs.chromium.org/chromium/src/content/test/test_blink_web_unit_test_support.cc?q=test_blink_web&sq=package:chromium&l=110
> (It looks really weird that we're creating a dummy message loop before calling
> blink::initialize.)