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This reverts commit 1ac9a1da.

Reason for revert: Breaking UI

Original change's description:
> Change ButtonPressed overrides to callbacks: c/b/ui/views/sharesheet/
>
> This allows converting |targets_| from a member to a parameter that's
> passed around.  I also did some other cleanup while there.
>
> Bug: 772945
> Change-Id: Iaa7f70594ba109d1598da3566856e90c8b09629d
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2481246
> Commit-Queue: Melissa Zhang <melzhang@chromium.org>
> Reviewed-by: Melissa Zhang <melzhang@chromium.org>
> Auto-Submit: Peter Kasting <pkasting@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#818324}

TBR=pkasting@chromium.org,melzhang@chromium.org

Change-Id: Ief930a27f44caef219bf782a7534ef8bc76d83ef
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 772945
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2483745Reviewed-by: Melissa Zhang <melzhang@chromium.org>
Commit-Queue: Melissa Zhang <melzhang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#818357}