OSWindow: small fix to the formatting of key event debugging BUG= Change-Id: I09ab785311ec0fa7f3eecb6484a965378a1fef0d Reviewed-on: https://chromium-review.googlesource.com/285835 Reviewed-by: Jamie Madill <jmadill@chromium.org> Tested-by: Corentin Wallez <cwallez@chromium.org>