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This means callers only need to call TearDown() if they need to do
something between teardown and destruction, which most don't; and they
cannot fail to call TearDown() (and thus leave objects alive).  So it's
both simpler and safer.

Bug: none
Change-Id: Iba7ad30e64dca62d060b8c572253117d7952db3b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2124979
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#754709}