Add WebUSB bindings and client interface

Source/modules/webusb/USBEndpoint.idl

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+enum USBDirection {
+    "in",
+    "out"
+};
+
+enum USBEndpointType {
+    "bulk",
+    "interrupt",
+    "isochronous"
+};
+
+[
+    Constructor(USBAlternateInterface alternate, octet endpointIndex),
+    RaisesException=Constructor,
+    GarbageCollected,
+    RuntimeEnabled=WebUSB,
+] interface USBEndpoint {
+    readonly attribute octet endpointNumber;
+    readonly attribute USBDirection direction;
+    readonly attribute USBEndpointType type;
+    readonly attribute unsigned long packetSize;
+};