Files app: Move common/js/externs.js to externs/*.

ui/file_manager/externs/directory_change_event.js

Index: ui/file_manager/externs/directory_change_event.js
diff --git a/ui/file_manager/externs/directory_change_event.js b/ui/file_manager/externs/directory_change_event.js
new file mode 100644
index 0000000000000000000000000000000000000000..d406268908dde8ba39e772bdb90eb4f66f3994f5
--- /dev/null
+++ b/ui/file_manager/externs/directory_change_event.js
@@ -0,0 +1,19 @@
+// Copyright 2016 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.
+
+/**
+ * @constructor
+ * @extends {Event}
+ * @struct
+ */
+var DirectoryChangeEvent = function() {};
+
+/** @type {DirectoryEntry} */
+DirectoryChangeEvent.prototype.previousDirEntry;
+
+/** @type {DirectoryEntry|FakeEntry} */
+DirectoryChangeEvent.prototype.newDirEntry;
+
+/** @type {boolean} */
+DirectoryChangeEvent.prototype.volumeChanged;