Bug 1574195 part 2. Switch key event interfaces to IDL mixins. r=masayuki

Differential Revision: https://phabricator.services.mozilla.com/D46521

--HG--
extra : moz-landing-system : lando

dom/bindings/Bindings.conf

@@ -440,6 +440,10 @@ DOMInterfaces = {
     'headerFile': 'DOMIntersectionObserver.h',
 },
 
+'KeyEvent' : {
+    'concrete': False,
+},
+
 'LegacyMozTCPSocket': {
     'headerFile': 'TCPSocket.h',
     'wrapperCache': False,

dom/webidl/KeyEvent.webidl

@@ -7,6 +7,11 @@
 // http://www.w3.org/TR/1999/WD-DOM-Level-2-19990923/events.html#Events-KeyEvent
 interface KeyEvent
 {
+  // It's all mixed in.
+};
+KeyEvent includes KeyEventMixin;
+
+interface mixin KeyEventMixin {
   const unsigned long DOM_VK_CANCEL         = 0x03;
   const unsigned long DOM_VK_HELP           = 0x06;
   const unsigned long DOM_VK_BACK_SPACE     = 0x08;

dom/webidl/KeyboardEvent.webidl

@@ -72,4 +72,4 @@ dictionary KeyboardEventInit : EventModifierInit
 };
 
 // Mozilla extensions
-KeyboardEvent implements KeyEvent;
+KeyboardEvent includes KeyEventMixin;