Add background-wide file metadata cache.

ui/file_manager/file_manager/background/js/global_metadata_cache.js

+// Copyright 2017 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.
+
+
+/**
+ * Maximum number of entries whose metadata can be cached.
+ * @const {number}
+ * @private
+ */
+var MAX_CACHED_METADATA = 10000;

[fukino, 2017/06/01 03:37:46]

Name it as MAX_CACHED_METADATA_ as it is private.

[tetsui, 2017/06/01 04:46:58]

Done.

+
+/**
+ * @private {!LRUCache<!Metadata>}
+ */
+var metadataCache = new LRUCache(MAX_CACHED_METADATA);

[fukino, 2017/06/01 03:37:46]

ditto

[tetsui, 2017/06/01 04:46:58]

Done.

+
+/**
+ * Returns metadata for the given FileEntry. Uses cached metadata if possible.
+ *
+ * @param {!FileEntry} entry
+ * @return {!Promise.<!Metadata>}
+ */
+function getEntryMetadata(entry) {
+  var entryURL = entry.toURL();
+  if (metadataCache.hasKey(entryURL)) {
+    return Promise.resolve(metadataCache.get(entryURL));
+  } else {
+    return new Promise(function(resolve, reject) {
+      entry.getMetadata(function(metadata) {
+        metadataCache.put(entryURL, metadata);
+        resolve(metadata);
+      }, reject);
+    });
+  }
+};