GitLab

+ 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 <huayinz@chromium.org>
Reviewed-by: Theresa <twellington@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606297}