[Spec] Elaborate on opaque-ads fenced frame size restrictions

[gtanzer]

The frame size is frozen at first navigation and coerced to the nearest allowed size.

[gtanzer]

Design doc (WIP): https://docs.google.com/document/d/1MVqxc2nzde3cJYIRC8vnXH-a4A6J4GQE-1vBuXhQsPE/edit#heading=h.67vioho3x0ux

The tentative list of options is:

Fixed sizes: 320x50 728x90 970x90 320x100 160x600 300x250 970x250 336x280 320x480 300x600 300x1050

Fixed-height flexible sizes (H): In mobile devices with viewport width vw, we also accept sizes (vw)xh, where h is one of {50, 100, 250}. For instance, in a device with viewport width 360, the sizes 360x50, 360x100, 360x250 are also valid sizes.

Fixed-aspect ratio flexible sizes (AR): In mobiles devices with viewport width vw, we also accept sizes (vw)x(vw/k), where k is one of {6.4, 3.2, 1.2, 2/3, 1/2}, rounding to the nearest integer (half up). For instance, in a device with viewport width 412, the sizes 412x64, 412x129, 412x343, 412x618 and 412x824 are also valid sizes.

[craigfrancis]

Related, it would be important for anything that isn’t a fixed sized ad, for the frame height to be set automatically, this has been raised for iframes: https://github.com/w3c/csswg-drafts/issues/1771