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And improve information provided by out-of-order destruction of task
scheduler and its provided constructs in unit tests.

(comes out of discussion on https://codereview.chromium.org/2852333004/)

BUG=708584, 689520
TBR=avi@chromium.org (documentation update in test_browser_thread_bundle.h)

Review-Url: https://codereview.chromium.org/2860063003
Cr-Commit-Position: refs/heads/master@{#469516}