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

Use the currently active (frontmost) window, which will usually be the
spawning Lacros window. Verify that it's still a ShellSurface, just in
case window activation has changed between when the user initiated the
file picker and ash-chrome has received the mojo message.

This isn't quite right -- in theory a non-active window could spawn a
file picker -- but it's better than a modal file picker that blocks
the whole desktop.

Bug: 1090587
Test: multiple lacros windows can have their own file picker children
Change-Id: I582cd534e49228f3da5ab67e5ad99812f1a48432
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2303681Reviewed-by: Erik Chen <erikchen@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#789719}