Web platform integration

[bakkot]

There are various web specs which use SetValueInBuffer directly (via WebIDL). These need to be updated and WPT tests written.

Also, there are specs which do a defensive copy of the bytes in a view which can be avoided when the buffer is immutable. This technically is an as-if optimization, but it's probably worth updating the WebIDL and consuming specs to call this out anyway because otherwise people will not know to make this optimization.

See discussion in Matrix.