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Select file dialogs are implemented in ash-chrome via
SelectFileDialogExtension, which requires an aura::Window owning window
for the dialog.

Top-level windows in lacros-chrome are represented in ash-chrome via
ShellSurface objects. Pass an ID for the shell surface over mojo from
lacros-chrome to ash-chrome and use it for window parenting.

The ID is a string: "org.chromium.lacros.<unguessable-token>", e.g.
"org.chromium.lacros.8B93BD96131616984961C7790B5E9A62". Lacros creates
an ID for each WaylandToplevelWindow and uses xdg_shell's set_app_id
method to set it. This is similar to how ARC++ encodes an Android
task ID in the app ID, e.g. "org.chromium.arc.12345".

The ID is sent as a string to provide future flexibility (in case we
need to encode different window types, like "lacros_popup") and because
it felt like an appropriate return type for the cross-platform method
ui::PlatformWindow::GetInternalWindowId().

Feature design doc at go/lacros-select-file

Bug: 1090587
Test: multiple simultaneous file open dialogs spawn/stack/close correctly
Change-Id: I7b2cdb934480f72f5dbb3523922d30cf5df0376a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2301119
Commit-Queue: James Cook <jamescook@chromium.org>
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: Robert Kroeger <rjkroege@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Reviewed-by: Erik Chen <erikchen@chromium.org>
Reviewed-by: Jorge Lucangeli Obes <jorgelo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791992}