fix(webrtc): Tell a page that takes the display without a held size to send it
Over WebRTC a controller beside a display's owner has its size held and
replayed when the display passes to it. A page that had sent no size the
server held took the display at the old owner's size and nothing asked
it for its own: test_multi_controller visible-wr failed that way once in
CI (800x600 kept through three checks, no "Resize requested" after the
hand-over). Over WebSockets the heir is told DISPLAY_OWNER and answers
with its size.
The server sends the heir a display_owner message when it replays no
size, and the page answers with its window's size.
webrtc::tests' owner_an_heir_that_held_no_size_is_told_it_owns_the_display
holds both cases (told without a held size, not told with one).
test_multi_controller: visible-wr 15/15 three times, visible 15/15,
webrtc 10/10, websockets 10/10. The unit tier (63) passes.