[resize-observer] devicePixelContentBoxSize is always present in ResizeObserverEntry whether or not it's being observed #6608
Description
https://drafts.csswg.org/resize-observer/#resize-observer-entry-interface shows that devicePixelContentBoxSize is always present in the entry.
The spec notes that observing device-pixel-content-box can be expensive because the UA has to compute it every time, but because devicePixelContentBoxSize is always present in ResizeObserverEntry, whether or not it's being observed, then the UA has to compute it on every callback anyway.
It would be better if the spec allowed devicePixelContentBoxSize to be optional, and only present when being observed.
cc @atotic