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This tears down DialogModel by through model_.reset() to simulate dialog
closing being synchronous. This gives simpler lifetimes for client code
as a synchronous delete is more predictable than an asynchronous task
from GetWidget()->Close() on its own which is asynchronous.

Note that from an implementation perspective the Widget teardown is not
synchronous, but this should not be a concern of DialogModel client
code.

Bug: 1106422
Change-Id: Ib35ec2389e78459ae98a4f901a35858a7fe97189
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2372929Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Peter Boström <pbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#801470}