exo: Flush gamepad added events properly.

Bug: 833986
Test: No delay in arc++ when adding new gamepad.
Change-Id: If314b5c7af90b7c308a13abf51bff5e5d27e9b77
Reviewed-on: https://chromium-review.googlesource.com/1015550
Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Commit-Queue: David Reveman <reveman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551452}

components/exo/wayland/server.cc

@@ -4399,6 +4399,7 @@ class WaylandGamingSeatDelegate : public GamingSeatDelegate {
 
     zcr_gaming_seat_v2_send_gamepad_added(gaming_seat_resource_,
                                           gamepad_resource);
+    wl_client_flush(wl_resource_get_client(gaming_seat_resource_));
 
     return gamepad_delegate;
   }