GitLab

Fill out the ash-chrome side of the implementation. Add support for
all dialog types (open, open folder, save, etc.).

For now, place the file dialog in the default ash window container.
Future CLs will add plumbing to use the Lacros window's exo
ShellSurface as the parent. Fix the ExtensionDialog code to allow a
parent aura::Window that isn't associated with a views::Widget.

1-page design doc at go/lacros-file-picker
Lacros project overview at go/lacros

Bug: 1090587
Test: manual, can open/save files with Ctrl-O/Ctrl-S
Change-Id: I54f461e92b3e5d75808bcf04ce903b003b12490b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2265191
Commit-Queue: James Cook <jamescook@chromium.org>
Reviewed-by: Erik Chen <erikchen@chromium.org>
Reviewed-by: Allen Bauer <kylixrd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#782846}