[ModalDialog] Add ModalDialogModel and ViewBinder

+ Added a ModalDialogModel and ModalDialogViewBinder
+ Migrate DownloadLocationDialog to use the model

Bug: 899318
Change-Id: I61d019cb4d707a65b09d30c2c3e185a63fd6caca
Reviewed-on: https://chromium-review.googlesource.com/c/1313763
Commit-Queue: Becky Zhou <[email protected]>
Reviewed-by: Theresa <[email protected]>
Cr-Commit-Position: refs/heads/master@{#606297}
14 files changed