Added plumbing for the availablechange event from Blink to WebPresentationClient.

Source/modules/presentation/AvailableChangeEventInit.idl

Index: Source/modules/presentation/AvailableChangeEventInit.idl
diff --git a/Source/modules/presentation/AvailableChangeEventInit.idl b/Source/modules/presentation/AvailableChangeEventInit.idl
new file mode 100644
index 0000000000000000000000000000000000000000..499c87e21588ced1a392b00082d9f28fc1f30611
--- /dev/null
+++ b/Source/modules/presentation/AvailableChangeEventInit.idl
@@ -0,0 +1,9 @@
+// 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.
+
+// https://w3c.github.io/presentation-api/#availablechangeevent
+
+dictionary AvailableChangeEventInit : EventInit {
+    boolean available = false;
+};