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This hooks up app-modal JS dialogs without adding any new APIs.

Most functionality is "free" i.e. already componentized, including
- alerts, prompts, and confirms (such as onbeforeunload)
- anti-spam suppressions ("prevent this webpage..." checkbox)

We will want to convert all except onBeforeUnload confirmations to
tab-modal, but we'll still need app-modal for onBeforeUnload, so this
is a reasonable stepping stone.

TBR=twellington@chromium.org

Bug: 1025256
Change-Id: I66575619dbfafcc0a575082c23f1e567568c29d5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1979154
Commit-Queue: Evan Stade <estade@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Clark DuVall <cduvall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#741168}