Add customElements.getName(constructor)

In CustomElements v0 you could use the `.name` field to get the defined
tag name, but CustomElements v1 does not offer such a field. `localName`
can be used within an instance but this prohibits use in, for example,
static methods.

This was discussed in the WCCG F2F, tracked in
WICG/webcomponents#566. The conclusion of the
F2F was to write up a spec change and WPT tests.

source

@@ -70865,6 +70865,7 @@ document.body.append(parent);
 interface <dfn interface>CustomElementRegistry</dfn> {
   [<span>CEReactions</span>] undefined <span data-x="dom-CustomElementRegistry-define">define</span>(DOMString name, <span>CustomElementConstructor</span> constructor, optional <span>ElementDefinitionOptions</span> options = {});
   (<span>CustomElementConstructor</span> or undefined) <span data-x="dom-CustomElementRegistry-get">get</span>(DOMString name);
+  DOMString? <span data-x="dom-CustomElementRegistry-getName">getName</span>(CustomElementConstructor constructor);
   Promise&lt;<span>CustomElementConstructor</span>&gt; <span data-x="dom-CustomElementRegistry-whenDefined">whenDefined</span>(DOMString name);
   [<span>CEReactions</span>] undefined <span data-x="dom-CustomElementRegistry-upgrade">upgrade</span>(<span>Node</span> root);
 };
@@ -70897,8 +70898,8 @@ dictionary <dfn dictionary>ElementDefinitionOptions</dfn> {
    <dt><code data-x=""><var>window</var>.<span data-x="dom-window-customElements">customElements</span>.<span subdfn data-x="dom-CustomElementRegistry-define">define</span>(<var>name</var>,
    <var>constructor</var>)</code></dt>
 
-   <dd>Defines a new <span>custom element</span>, mapping the given name to the given constructor as
-   an <span>autonomous custom element</span>.</dd>
+   <dd>Defines a new <span>custom element</span>, mapping the given name to the given constructor
+   as an <span>autonomous custom element</span>.</dd>
 
 
    <dt><code data-x=""><var>window</var>.<span data-x="dom-window-customElements">customElements</span>.<span data-x="dom-CustomElementRegistry-define">define</span>(<var>name</var>, <var>constructor</var>,
@@ -70915,6 +70916,11 @@ dictionary <dfn dictionary>ElementDefinitionOptions</dfn> {
    <dd>Retrieves the <span>custom element constructor</span> defined for the given <span data-x="concept-custom-element-definition-name">name</span>. Returns undefined if there is no
    <span>custom element definition</span> with the given <span data-x="concept-custom-element-definition-name">name</span>.</dd>
 
+   <dt><code data-x=""><var>window</var>.<span data-x="dom-window-customElements">customElements</span>.<span subdfn data-x="dom-CustomElementRegistry-getName">getName</span>(<var>constructor</var>)</code></dt>
+
+   <dd>Retrieves the given name for a <span>custom element</span> defined for the given <span data-x="concept-custom-element-definition-constructor">constructor</span>. Returns null if there is no
+   <span>custom element definition</span> with the given <span data-x="concept-custom-element-definition-constructor">constructor</span>.</dd>
+
    <dt><code data-x=""><var>window</var>.<span data-x="dom-window-customElements">customElements</span>.<span subdfn data-x="dom-CustomElementRegistry-whenDefined">whenDefined</span>(<var>name</var>)</code></dt>
 
    <dd>Returns a promise that will be fulfilled with the <span>custom element</span>'s constructor
@@ -71154,6 +71160,19 @@ dictionary <dfn dictionary>ElementDefinitionOptions</dfn> {
    <li><p>Otherwise, return undefined.</p></li>
   </ol>
 
+  <p>The <dfn method for="CustomElementRegistry"><code
+  data-x="dom-CustomElementRegistry-getName">getName(<var>constructor</var>)</code></dfn> method
+  steps are:</p>
+
+  <ol>
+   <li><p>If this <code>CustomElementRegistry</code> contains an entry with <span
+   data-x="concept-custom-element-definition-constructor">constructor</span>
+   <var>constructor</var>, then return that entry's <span
+   data-x="concept-custom-element-definition-name">name</span>.</p></li>
+
+   <li><p>Return null.</p></li>
+  </ol>
+
   <p>When invoked, the <dfn method for="CustomElementRegistry"><code
   data-x="dom-CustomElementRegistry-whenDefined">whenDefined(<var>name</var>)</code></dfn> method
   must run these steps:</p>
@@ -139077,6 +139096,7 @@ INSERT INTERFACES HERE
   葛依寧 (Kat Hackett),
   Kathy Walton,
   河童エクマ（Kawarabe Ecma）<!-- heppokofrontend on GitHub -->
+  Keith Cirkel, <!-- keithamus on GitHub -->
   Keith Rollin,
   Keith Yeung,
   Kelly Ford,