Add background-wide file metadata cache.

ui/file_manager/file_manager/background/js/import_history.js

Index: ui/file_manager/file_manager/background/js/import_history.js
diff --git a/ui/file_manager/file_manager/background/js/import_history.js b/ui/file_manager/file_manager/background/js/import_history.js
index aba2c16f454cc3be96b0e405c3f1d4a5d2e19c09..46558772ba0928b41daa170f04be4509fdef40b4 100644
--- a/ui/file_manager/file_manager/background/js/import_history.js
+++ b/ui/file_manager/file_manager/background/js/import_history.js
@@ -1011,3 +1011,37 @@ importer.RuntimeHistoryLoader.prototype.addHistoryLoadedListener =
     function(listener) {
   this.historyResolver_.promise.then(listener);
 };
+
+/**
+ * @param {!FileEntry} fileEntry
+ * @return {!Promise<string>} Resolves with a "hashcode" consisting of
+ *     just the last modified time and the file size.
+ */
+importer.createMetadataHashcode = function(fileEntry) {
+  return new Promise(
+             /**
+              * @param {function()} resolve
+              * @param {function()} reject
+              * @this {importer.PersistentImportHistory}
+              */
+             function(resolve, reject) {
+               metadataProxy.getEntryMetadata(fileEntry).then(
+                   /**
+                    * @param {!Object} metadata
+                    * @return {!Promise<string>}
+                    * @this {importer.PersistentImportHistory}
+                    */
+                   function(metadata) {
+                     if (!('modificationTime' in metadata)) {
+                       reject('File entry missing "modificationTime" field.');
+                     } else if (!('size' in metadata)) {
+                       reject('File entry missing "size" field.');
+                     } else {
+                       var secondsSinceEpoch = importer.toSecondsFromEpoch(
+                           metadata.modificationTime);
+                       resolve(secondsSinceEpoch + '_' + metadata.size);
+                     }
+                   }.bind(this));
+             }.bind(this))
+      .catch(importer.getLogger().catcher('importer-common-create-hashcode'));
+};