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This callback will be needed to determine if we should reparent web
contents into an app window on macOS. For the moment, leave the
callback unused in all instances.

Bug: 915571
Change-Id: Ib86415a7a3d1981db817c39dfe7bb3a8b23ed571
Reviewed-on: https://chromium-review.googlesource.com/c/1487917Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Reviewed-by: Alexey Baskakov <loyso@chromium.org>
Commit-Queue: ccameron <ccameron@chromium.org>
Cr-Commit-Position: refs/heads/master@{#635648}