It can happen that we leave AC mode after a new image is received from the web process but before it's committed. It can also happen that we receive a new image from the web process right after we have left AC mode. In both cases we need to complete the frame to ensure we don't leave the wayland compositor waiting forever for a frame to be completed.
Created attachment 370938 [details]
Committed r245957: <https://trac.webkit.org/changeset/245957>