[test::TaskEnvironment] Rename Thread::TaskEnvironment to avoid confusion

Precursor to https://chromium-review.googlesource.com/c/chromium/src/+/1747171

R=dcheng@chromium.org

Bug: 992483
Change-Id: I0bd990acbfc97ae95b9faaba1ecab682f85150c3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1747190
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Commit-Queue: Sami Kyöstilä <skyostil@chromium.org>
Cr-Commit-Position: refs/heads/master@{#685964}
15 files changed