Add optional submitter argument to FormData constructor

Tests: web-platform-tests/wpt#37895.

Closes #262.

xhr.bs

@@ -9,6 +9,11 @@ Translation: ja https://triple-underscore.github.io/XHR-ja.html
 Translate IDs: enumdef-xmlhttprequestresponsetype xmlhttprequestresponsetype,dictdef-progresseventinit progresseventinit,typedefdef-formdataentryvalue formdataentryvalue
 </pre>
 
+<!-- Temporary: https://github.com/w3c/webdriver-bidi/issues/358 -->
+<pre class=link-defaults>
+spec:webdriver-bidi; type:dfn; text:event
+</pre>
+
 <pre class=anchors>
 urlPrefix: https://w3c.github.io/DOM-Parsing/; spec: dom-parsing
     type: dfn; text: fragment serializing algorithm; url: dfn-fragment-serializing-algorithm
@@ -1609,7 +1614,7 @@ typedef (File or USVString) FormDataEntryValue;
 
 [Exposed=(Window,Worker)]
 interface FormData {
-  constructor(optional HTMLFormElement form);
+  constructor(optional HTMLFormElement form, optional HTMLElement? submitter = null);
 
   undefined append(USVString name, USVString value);
   undefined append(USVString name, Blob blobValue, optional USVString filename);
@@ -1635,16 +1640,27 @@ interface FormData {
 moved to the HTML Standard. [[HTML]]
 
 <p>The
-<dfn id=dom-formdata constructor for=FormData lt="FormData(form)"><code>new FormData(<var>form</var>)</code></dfn>
+<dfn id=dom-formdata constructor for=FormData lt="FormData(form)"><code>new FormData(<var>form</var>, <var>submitter</var>)</code></dfn>
 constructor steps are:
 
 <ol>
  <li>
   <p>If <var>form</var> is given, then:
 
   <ol>
+   <li>
+    <p>If <var>submitter</var> is non-null, then:
+
+    <ol>
+     <li><p>If <var>submitter</var> is not a <a>submit button</a>, then <a>throw</a> a
+     {{TypeError}}.
+
+     <li><p>If <var>submitter</var>'s <a>form owner</a> is not <var>form</var>, then <a>throw</a> a
+     "{{NotFoundError!!exception}}" {{DOMException}}.
+    </ol>
+
    <li><p>Let <var>list</var> be the result of <a>constructing the entry list</a> for
-   <var>form</var>.
+   <var>form</var> and <var>submitter</var>.
 
    <li><p>If <var>list</var> is null, then <a>throw</a> an "{{InvalidStateError!!exception}}"
    {{DOMException}}.