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This CL updates the JS dialog egtests to work with both modal and non-
modal dialog implementations in preparation for updating the non-modal
dialog feature to be enabled by default.

This required introducing a11y identifier functionality to the
AlertConsumer protocol to allow the tests to properly wait for the
dialogs to be displayed.

Bug: none
Change-Id: I26010b73501442db729f8c561d925d70b8c5eca5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1925775
Commit-Queue: Kurt Horimoto <kkhorimoto@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#718384}