Add a media scanner convenience class.

ui/file_manager/file_manager/background/js/media_scanner.js

Index: ui/file_manager/file_manager/background/js/media_scanner.js
diff --git a/ui/file_manager/file_manager/background/js/media_scanner.js b/ui/file_manager/file_manager/background/js/media_scanner.js
new file mode 100644
index 0000000000000000000000000000000000000000..7c744b2e214768e5bcc90d94c3b31f1c1f8f2c6b
--- /dev/null
+++ b/ui/file_manager/file_manager/background/js/media_scanner.js
@@ -0,0 +1,65 @@
+// Copyright 2014 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.
+
+/**
+ * Recursively scans through a list of given files and directories, and creates
+ * a list of media files.
+ *
+ * @param {!Array.<!Entry>} entries A list of file and directory entries.  File
+ *     entries are added directly to the media list; directory entries are
+ *     recursively traversed to find files, which are added to the media list.
+ * @constructor
+ */
+function MediaScanner(entries) {
+  /** @private {!Promise<!Array<!FileEntry>>} */
+  this.filesPromise_ = this.scan_(entries);
+}
+
+/**
+ * Scans a list of directory and file entries, returning image and video files.
+ * @param {!Array<!Entry>} entries
+ */
+MediaScanner.prototype.scan_ = function(entries) {
+  /**
+   * Returns files and directories found under the given Entry.
+   * @param {!Entry} entry
+   * @return {!Promise<!Array<!Entry>>}
+   */
+  var scanRecurse = function(entry) {
+    return new Promise(function(resolve, reject) {
+      fileOperationUtil.resolveRecursively(entry, resolve, reject);
+    });
+  };
+
+  /**
+   * Flattens a nested list of Entries.
+   * @param {!Array<!Array<!Entry>>} array
+   * @return {!Array<!Entry>}
+   */
+  var flatten = function(array) {
+    return array.reduce(function(prev, curr) {
+      return prev.concat(curr);
+    }, []);
+  };
+
+  /**
+   * Filters non-image and non-video files out of the given list.
+   * @param {!Array<!Entry>} array
+   * @return {!Array<!Entry>}
+   */
+  var filter = function(array) {
+    return array.filter(FileType.isImageOrVideo);
+  };
+
+  return Promise.all(entries.map(scanRecurse))
+      .then(flatten)
+      .then(filter);
+};
+
+/**
+ * @return {!Promise<!Array<!FileEntry>>}
+ */
+MediaScanner.prototype.getFiles = function() {
+  return this.filesPromise_;
+};