Add toggleAttribute() to Element

Tests: see https://bugzilla.mozilla.org/show_bug.cgi?id=1469592. Will be upstreamed soonish.

Fixes #461.

dom.bs

@@ -5789,6 +5789,7 @@ interface Element : Node {
   [CEReactions] void setAttributeNS(DOMString? namespace, DOMString qualifiedName, DOMString value);
   [CEReactions] void removeAttribute(DOMString qualifiedName);
   [CEReactions] void removeAttributeNS(DOMString? namespace, DOMString localName);
+  [CEReactions] boolean toggleAttribute(DOMString qualifiedName, optional boolean force);
   boolean hasAttribute(DOMString qualifiedName);
   boolean hasAttributeNS(DOMString? namespace, DOMString localName);
 
@@ -6464,6 +6465,14 @@ namespace.</p>
  <dd><p>Removes <var>element</var>'s <a>attribute</a> whose <a for=Attr>namespace</a> is
  <var>namespace</var> and <a for=Attr>local name</a> is <var>localName</var>.
 
+ <dt><code><var>element</var> . <a method for=Element lt=toggleAttribute()>toggleAttribute</a>(<var>qualifiedName</var> [, <var>force</var>])</code>
+ <dd>
+  <p>If <var>force</var> is not given, "toggles" <var>qualifiedName</var>, removing it if it is
+  present and adding it if it is not present. If <var>force</var> is true, adds
+  <var>qualifiedName</var>. If <var>force</var> is false, removes <var>qualifiedName</var>.
+
+  <p>Returns true if <var>qualifiedName</var> is now present, and false otherwise.
+
  <dt><code><var>element</var> . <a method for=Element lt=hasAttribute()>hasAttribute</a>(<var>qualifiedName</var>)</code>
  <dd><p>Returns true if <var>element</var> has an <a>attribute</a> whose
  <a for=Attr>qualified name</a> is <var>qualifiedName</var>, and false otherwise.
@@ -6577,6 +6586,42 @@ when invoked, must run these steps:
  otherwise.
 </ol>
 
+<p>The <dfn method for=Element><code>toggleAttribute(<var>qualifiedName</var>, <var>force</var>)</code></dfn>
+method, when invoked, must run these steps:
+
+<ol>
+ <li><p>If <var>qualifiedName</var> does not match the <code><a type>Name</a></code> production in
+ XML, then <a>throw</a> an "{{InvalidCharacterError!!exception}}" {{DOMException}}.
+
+ <li><p>If the <a>context object</a> is in the <a>HTML namespace</a> and its
+ <a for=Node>node document</a> is an <a>HTML document</a>, then set <var>qualifiedName</var> to
+ <var>qualifiedName</var> in <a>ASCII lowercase</a>.
+
+ <li><p>Let <var>attribute</var> be the first <a>attribute</a> in the <a>context object</a>'s
+ <a for=Element>attribute list</a> whose <a for=Attr>qualified name</a> is <var>qualifiedName</var>,
+ and null otherwise.
+ <!-- This is step 2 of "get an attribute by name", modified as appropriate -->
+
+ <li>
+  <p>If <var>attribute</var> is null, then:
+
+  <ol>
+   <li><p>If <var>force</var> is not given or is true, create an <a>attribute</a> whose
+   <a for="Attr">local name</a> is <var>qualifiedName</var>, <a for=Attr>value</a> is the empty
+   string, and <a for=Node>node document</a> is the <a>context object</a>'s
+   <a for=Node>node document</a>, then <a lt="append an attribute">append</a> this <a>attribute</a>
+   to the <a>context object</a>, and then return true.
+
+   <li><p>Return false.
+  </ol>
+
+ <li><p>Otherwise, if <var>force</var> is not given or is false,
+ <a lt="remove an attribute by name">remove an attribute</a> given <var>qualifiedName</var> and the
+ <a>context object</a>, and then return false.
+
+ <li><p>Return true.
+</ol>
+
 <p>The
 <dfn method for=Element><code>hasAttributeNS(<var>namespace</var>, <var>localName</var>)</code></dfn>
 method, when invoked, must run these steps:
@@ -9629,7 +9674,7 @@ method, when invoked, must run these steps:
    <var>token</var> from <a>context object</a>'s <a>token set</a>, run the <a>update steps</a> and
    return false.
 
-   <li><p>Otherwise, return true.
+   <li><p>Return true.
   </ol>
 
  <li><p>Otherwise, if <var>force</var> not given or is true, <a for=set>append</a> <var>token</var>
@@ -9915,6 +9960,7 @@ Joe Kesselman,
 John Atkins,
 John Dai,
 Jonas Sicking,
+Jonathan Kingston,
 Jonathan Robie,
 Joris van der Wel,
 Joshua Bell,