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This introduces DialogModelField::AsButton, etc. instead of having
DialogModelHost and DialogModel both downcast.

Also adds DialogModel::AddField to make sure OnFieldAdded() is called
from all call sites that add a field.

Bug: 1106422
Change-Id: I512f5df9cf642314de7bde5b9ef4fe1d7c3efedc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2363870
Commit-Queue: Peter Boström <pbos@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Auto-Submit: Peter Boström <pbos@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#799439}