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

ui/file_manager/externs/entries_changed_event.js

Index: ui/file_manager/externs/entries_changed_event.js
diff --git a/ui/file_manager/externs/entries_changed_event.js b/ui/file_manager/externs/entries_changed_event.js
new file mode 100644
index 0000000000000000000000000000000000000000..b646bfa1c0042249b228c4d82e2f9e90d8b6fef1
--- /dev/null
+++ b/ui/file_manager/externs/entries_changed_event.js
@@ -0,0 +1,15 @@
+// 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}
+ */
+var EntriesChangedEvent = function() {};
+
+/** @type {util.EntryChangedKind} */
+EntriesChangedEvent.prototype.kind;
+
+/** @type {Array<!Entry>} */
+EntriesChangedEvent.prototype.entries;