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As discussed @
https://chromium-review.googlesource.com/c/chromium/src/+/1749538/6/ash/test/ash_test_base.h#84

Usage of explicit everywhere also avoids implicit copy construction
and is the reason for changes to a few unit tests that previously
used operator= to initialize their task environments.

TBR=dcheng@chromium.org
(for widespread mechanical change)

Bug: 992483
Change-Id: I321576674f3b387a6f5069e8e71eebf41c98e7ed
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1758338Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#689098}