Add USBConnectionEvent to webusb module.

Source/modules/webusb/USBConnectionEvent.idl

Index: Source/modules/webusb/USBConnectionEvent.idl
diff --git a/Source/modules/webusb/USBConnectionEvent.idl b/Source/modules/webusb/USBConnectionEvent.idl
new file mode 100644
index 0000000000000000000000000000000000000000..3080f98b07a0ad416f6939470e0b0b5652ae3e8c
--- /dev/null
+++ b/Source/modules/webusb/USBConnectionEvent.idl
@@ -0,0 +1,12 @@
+// 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.
+
+// http://reillyeon.github.io/webusb/#events
+
+[
+    Constructor(DOMString type, optional USBConnectionEventInit eventInitDict),
+    RuntimeEnabled=WebUSB,
+] interface USBConnectionEvent : Event {
+    readonly attribute USBDevice device;
+};