CardUnmaskPrompt is modified to use ModalDialogView instead of
AlertDialogs. This is part of an afford to convert all instances of
modal dialogs to use ModalDialogView.

Bug: 836366
Change-Id: I3a0b785f89b1e49768578529ba2c4a8302e8e83e
Reviewed-on: https://chromium-review.googlesource.com/1059709
Commit-Queue: Amirhossein Simjour <asimjour@chromium.org>
Reviewed-by: Theresa <twellington@chromium.org>
Reviewed-by: Mathieu Perreault <mathp@chromium.org>
Reviewed-by: Michael Wasserman <msw@chromium.org>
Reviewed-by: Becky Zhou <huayinz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#565998}