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This code jumped through a lot of complex hoops to allow subclasses to
provide a TestEnvironment of their own.  But no subclasses actually
want a test environment with a different lifetime than the parent's.
So just allow people to pass in a TestEnvironment instance instead.

This, coupled with providing an accessor for calling methods, allows
making the TaskEnvironment private instead of protected.

Bug: none
Change-Id: I923f5b232eb8fc01cc445e4d51cf45707d167ae7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2126093
Auto-Submit: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#754560}