Start to use new metadata item in ImageEncoder.

ui/file_manager/gallery/js/gallery_item_unittest.js

 // Copyright 2015 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.
 
 /**
  * Mock of ImageUtil.
  */
 var ImageUtil = {
   getMetricName: function() {},
   metrics: {
@@ skipping 45 matching lines @@
     },
     clear: function(entries, type) {
       fetchedMediaCleared = true;
     }
   };
   var item = new Gallery.Item(
       entry,
       {isReadOnly: false},
       {name: 'oldMetadata'},
       metadataCache,
+      // Mock of FileSystemMetadata.
+      {
+        get: function() {
+          return Promise.resolve({});
+        },
+        notifyEntriesChanged: function() {}
+      },
       /* original */ true);
   assertEquals('oldMetadata', item.getMetadata().name);
   assertFalse(fetchedMediaCleared);
   reportPromise(
       new Promise(item.saveToFile.bind(
           item,
           {getLocationInfo: function() { return {}; }},
           null,
           true,
           document.createElement('canvas'))).then(function() {
             assertEquals('newMetadata', item.getMetadata().name);
             assertTrue(fetchedMediaCleared);
           }), callback);
 }