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This is needed due to a specific requirements for integration with the
extensions Management API, which needs to first create the dialog, then
create the callback that takes ownership of the dialog instance, then
show the dialog.

That order isn't possible when the dialog's callback is set in
its constructor.

Also remove unused ShowParentPermissionDialog...() fns from
SupervisedUserService since we need to create the dialogs directly to
achieve the pattern described above.

Bug:957832

Change-Id: I9852a4ecbe12babfadd3a806513713d2085b3d21
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2088952
Auto-Submit: Dan S <danan@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Dan S <danan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#747827}