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We need call wl_client_flush in order to ensure that these events
are transmitted to the client.

Bug: 824449
Test: gtk-demo --gdk-debug=misc show that output event is received
Change-Id: I18e74339b306ed56c85563628fc8b6af7886fd82
Reviewed-on: https://chromium-review.googlesource.com/1014908Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Commit-Queue: David Reveman <reveman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551397}