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Add the skeleton for a mojo API for open/save file dialogs, roughly
following the existing SelectFileDialog C++ interface.

Add SelectFileDialogLacros, which lives in lacros-chrome and binds
to a remote SelectFile interface in ash-chrome. The class lives in
//ui/shell_dialogs to be close to other per-platform select dialog
wrappers, as well as to demonstrate how a lacros mojo API can be
called from a component outside src/chrome.

Add a stub SelectFileImpl in ash-chrome. For now this just returns
a single file immediately. Follow-up CLs will actually open a dialog,
using the existing file manager-backed dialog used by ash.

1-page design doc at go/lacros-file-picker

Bug: 1090587
Test: manual, hit Ctrl-O and /etc/lsb-release is loaded in a tab
Change-Id: I9ebd9ff25f8aa32deac5eabb85f40a37e4b72856
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2261405
Commit-Queue: James Cook <jamescook@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Greg Kerr <kerrnel@chromium.org>
Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#782801}