Auto merge of #21074 - jonathanKingston:toggle-attribute, r=Manishearth

Implement support for Element.toggleAttribute

Copy implementation following from [Firefox implementation](https://bugzilla.mozilla.org/show_bug.cgi?id=1469592) of soon to be standardised [Element.toggleAttribute](https://github.com/whatwg/dom/pull/656). [Other implementers agreed to ship](https://github.com/whatwg/dom/issues/461)

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components/script/dom/webidls/Element.webidl

@@ -41,6 +41,8 @@ interface Element : Node {
   [Pure]
   DOMString? getAttributeNS(DOMString? namespace, DOMString localName);
   [CEReactions, Throws]
+  boolean toggleAttribute(DOMString name, optional boolean force);
+  [CEReactions, Throws]
   void setAttribute(DOMString name, DOMString value);
   [CEReactions, Throws]
   void setAttributeNS(DOMString? namespace, DOMString name, DOMString value);