Page's join_layout can report incorrect results

[jdm]

As described in #3358 (comment), the handle_reflow_complete logic in ScriptTask looks like it properly handles the case of waiting for the last-known script-initiated reflow to complete, but Page's join_layout does not. We should either make join_layout aware of this (by storing a page's last reflow ID in the Page itself and migrating the logic) or force the compositor to send reflow commands via the script task.

[jdm]

What if we reset the join_layout_port every time content initiates a reflow and send the new sender object with each request? Then join_layout can listen on the newest port, layout can discard any send errors, and compositor-initiated reflow won't cause any spurious notifications to be sent to content.