Gallery: Store image caches in Gallery items.

ui/file_manager/gallery/js/gallery_item.js

Index: ui/file_manager/gallery/js/gallery_item.js
diff --git a/ui/file_manager/gallery/js/gallery_item.js b/ui/file_manager/gallery/js/gallery_item.js
index 18c9c95043050a6a1e5a065cb6ac44b8a3cb17f7..7f146ada36937acf8534e304108fb33ece53139a 100644
--- a/ui/file_manager/gallery/js/gallery_item.js
+++ b/ui/file_manager/gallery/js/gallery_item.js
@@ -27,10 +27,34 @@ Gallery.Item = function(entry, metadata, metadataCache, original) {
 
   /**
    * @type {MetadataCache}
+   * @private
    */
   this.metadataCache_ = metadataCache;
 
   /**
+   * The content cache is used for prefetching the next image when going through
+   * the images sequentially. The real life photos can be large (18Mpix = 72Mb
+   * pixel array) so we want only the minimum amount of caching.
+   * @type {Canvas}
+   */
+  this.screenImage = null;
+
+  /**
+   * We reuse previously generated screen-scale images so that going back to a
+   * recently loaded image looks instant even if the image is not in the content
+   * cache any more. Screen-scale images are small (~1Mpix) so we can afford to
+   * cache more of them.
+   * @type {Canvas}
+   */
+  this.contentImage = null;
+
+  /**
+   * Last accessed date to be used for selecting items whose cache are evicted.
+   * @type {Date}
+   */
+  this.lastAccessed = Date.now();
+
+  /**
    * @type {boolean}
    * @private
    */