Implement the InputEvent

Add WebIDL, from UI Events working draft.
Implement script type for event.

Note that actually firing InputEvents is not part of this commit.

components/script/dom/webidls/InputEvent.webidl

@@ -0,0 +1,21 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+/*
+ * The origin of this IDL file is
+ * https://w3c.github.io/uievents/#idl-inputevent
+ *
+ */
+
+// https://w3c.github.io/uievents/#idl-inputevent
+[Constructor(DOMString type, optional InputEventInit eventInitDict)]
+interface InputEvent : UIEvent {
+  readonly attribute DOMString? data;
+  readonly attribute boolean isComposing;
+};
+
+// https://w3c.github.io/uievents/#idl-inputeventinit
+dictionary InputEventInit : UIEventInit {
+  DOMString? data = null;
+  boolean isComposing = false;
+};