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This CL is based on previous Nick's work (https://crrev.com/c/2051187)
and fixed modality for gtk file dialogs by leveraging xdg-foreign
protocol extension.

XdgForeignWrapper has been added to ease usage of the protocol.

Bug: 963419, 1008755
Change-Id: I68eb5818a6bb8e7dbf28a13f46e86926dc4cfdda
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2397575
Commit-Queue: Maksim Sisov (GMT+3) <msisov@igalia.com>
Reviewed-by: Nick Yamane <nickdiego@igalia.com>
Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#806078}