Update the “Link type "serviceworker"” section (#1110)
* Add new section “Declaring a "serviceworker" Link header”
* Drop normative requirement to fire an error if link[workertype] value
isn’t a valid worker type; instead just abort.
* Drop the partial interface IDL for the HTMLLinkElement interface (the
definitions for the IDL attributes have already been merged directly
into the HTMLLinkElement interface IDL definition in the HTML spec).
Fixes #1073
