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These methods express that the caller is passing in ownership of the
delegate, and assert that the delegate believes it is owned by the
widget.

This change also corrects a flaw in ownership handling in an ash
unittest: some of the created delegates were passed around as raw
pointers, and one was sort of leaked (not actually leaked, but
destroyed in an extremely non-obvious way).

Bug: 1075649
Change-Id: I655dfe3e2466032b85247341f43b1b2d75c0b561
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2446015Reviewed-by: Peter Boström <pbos@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#813416}