Tweak <link>.workerType reflection

9c7da49 removed the idea of using
enumerations in the IDL to case-sensitively reflect properties, but
forgot to switch <link>'s workerType attribute to DOMString. This fixes
that.

Additionally, this introduces the invalid state as the invalid value
default, so that service workers' processing model can ignore invalid
values instead of treating them as classic scripts (similar to
<script>'s type="" attribute).

source

@@ -12929,7 +12929,7 @@ interface <dfn>HTMLLinkElement</dfn> : <span>HTMLElement</span> {
   [SameObject, PutForwards=<span data-x="dom-DOMTokenList-value">value</span>] readonly attribute <span>DOMTokenList</span> <span data-x="dom-link-sizes">sizes</span>;
   [<span>CEReactions</span>] attribute DOMString <span data-x="dom-link-referrerPolicy">referrerPolicy</span>;
   [<span>CEReactions</span>] attribute USVString <span data-x="dom-link-scope">scope</span>;
-  [<span>CEReactions</span>] attribute <span>WorkerType</span> <span data-x="dom-link-workertype">workerType</span>;
+  [<span>CEReactions</span>] attribute DOMString <span data-x="dom-link-workertype">workerType</span>;
   [<span>CEReactions</span>] attribute DOMString <span data-x="dom-link-updateviacache">updateViaCache</span>;
 };
 <span>HTMLLinkElement</span> implements <span>LinkStyle</span>;</pre>
@@ -13150,9 +13150,10 @@ interface <dfn>HTMLLinkElement</dfn> : <span>HTMLElement</span> {
   <span>service worker registration</span>. The attribute's keywords are <code
   data-x="">classic</code> and <code data-x="">module</code>, which map to the respective states
   <i>classic</i> and <i>module</i>. The <span>missing value default</span> is the <i>classic</i>
-  state. The attribute must not be specified on <code>link</code> elements that do not have a <code
-  data-x="attr-link-rel">rel</code> attribute that contains the <code
-  data-x="rel-serviceworker">serviceworker</code> keyword.</p>
+  state. There is an additional state, not represented by a keyword: the <i>invalid</i> state. The
+  <span>invalid value default</span> is the <i>invalid</i> state. The attribute must not be
+  specified on <code>link</code> elements that do not have a <code data-x="attr-link-rel">rel</code>
+  attribute that contains the <code data-x="rel-serviceworker">serviceworker</code> keyword.</p>
 
   <hr>
 
@@ -13209,7 +13210,7 @@ interface <dfn>HTMLLinkElement</dfn> : <span>HTMLElement</span> {
 
   <p>The <dfn><code data-x="dom-link-workerType">workerType</code></dfn> IDL attribute must
   <span>reflect</span> the <code data-x="attr-link-workertype">workertype</code> content
-  attribute.</p>
+  attribute, <span>limited to only known values</span>.</p>
 
   <h5>Processing the <code data-x="attr-link-media">media</code> attribute</h5>