Implement Windows file dialog.

On Windows, we need to run the dialog in a separate process run with
user privileges.

Bug: 679313
Change-Id: I54b3a1288f72ec9c11964fac379ef72e0589e8a4
Reviewed-on: https://chromium-review.googlesource.com/c/1434794
Reviewed-by: Joe Downing <joedow@chromium.org>
Commit-Queue: Erik Jensen <rkjnsn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#629385}
10 files changed