Call URL.revokeObjectURL for created URLs (#1858)

* Call URL.revokeObjectURL for created URLs

* Refactor image usage

ext/js/language/dictionary-importer.js

@@ -424,9 +424,10 @@ class DictionaryImporter {
         }
 
         // Load image data
-        let image;
+        let width;
+        let height;
         try {
-            image = await this._loadImageBase64(mediaType, content);
+            ({width, height} = await this._getImageResolution(mediaType, content));
         } catch (e) {
             throw createError('Could not load image');
         }
@@ -436,8 +437,8 @@ class DictionaryImporter {
             dictionary,
             path,
             mediaType,
-            width: image.naturalWidth,
+            width,
-            height: image.naturalHeight,
+            height,
             content
         };
         media.set(path, mediaData);
@@ -461,22 +462,29 @@ class DictionaryImporter {
     }
 
     /**
-     * Attempts to load an image using a base64 encoded content and a media type.
+     * Attempts to load an image using a base64 encoded content and a media type
+     * and returns its resolution.
      * @param mediaType The media type for the image content.
      * @param content The binary content for the image, encoded in base64.
-     * @returns A Promise which resolves with an HTMLImageElement instance on
+     * @returns A Promise which resolves with {width, height} on success,
-     *   successful load, otherwise an error is thrown.
+     *   otherwise an error is thrown.
      */
-    _loadImageBase64(mediaType, content) {
+    _getImageResolution(mediaType, content) {
         return new Promise((resolve, reject) => {
             const image = new Image();
             const eventListeners = new EventListenerCollection();
-            eventListeners.addEventListener(image, 'load', () => {
+            const cleanup = () => {
+                image.removeAttribute('src');
+                URL.revokeObjectURL(url);
                 eventListeners.removeAllEventListeners();
-                resolve(image);
+            };
+            eventListeners.addEventListener(image, 'load', () => {
+                const {naturalWidth: width, naturalHeight: height} = image;
+                cleanup();
+                resolve({width, height});
             }, false);
             eventListeners.addEventListener(image, 'error', () => {
-                eventListeners.removeAllEventListeners();
+                cleanup();
                 reject(new Error('Image failed to load'));
             }, false);
             const blob = MediaUtil.createBlobFromBase64Content(content, mediaType);