[filelist] Block invalid FileList thumbnail loader requests early

ImageLoaderClient is throwing DOM exceptions while processing requests
{ImageLoaderRequest} that have no request URL. Odd, the request URL is
required (it keys the ImageLoader caches). Tracked the source of these
“no-url” requests down to the FileList thumbnail loader.

The FileList thumbnail loader sends ImageLoaderClient requests for the
file list entries while the user is scrolling the FileList. So it is a
good idea to avoid doing unnecessary work therein (it’s hot code).

Change the FileList thumbnail loader to setup the URL first and reject
early with an ImageLoaderResponse error if there is no URL to request.
Enjoy the faster scrolling, CPU and power savings, that result.

Bug: 965370
Change-Id: I5dd9a1390bc1f92010aea1b4ca2e8a50b67177df
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1695661Reviewed-by: Alex Danilo <adanilo@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#676225}

ui/file_manager/file_manager/foreground/js/thumbnail_loader.js

@@ -199,15 +199,15 @@ class ThumbnailLoader {
   }
 
   /**
-   * Loads thumbnail as data url. If data url of thumbnail can be fetched from
+   * Loads thumbnail as dataUrl. If the thumbnail dataUrl can be fetched from
    * metadata, this fetches it from it. Otherwise, this tries to load it from
    * thumbnail loader.
    * Compared with ThumbnailLoader.load, this method does not provide a
    * functionality to fit image to a box.
    *
-   * @param {ThumbnailLoader.FillMode} fillMode Only FIT and OVER_FILL is
-   *     supported. This takes effect only when external thumbnail source is
-   *     used.
+   * @param {ThumbnailLoader.FillMode} fillMode Only FIT and OVER_FILL are
+   *     supported. This takes effect only when an external thumbnail source
+   *     is used.
    * @return {!Promise<{data:string, width:number, height:number}>} A promise
    *     which is resolved when data url is fetched.
    *
@@ -219,12 +219,26 @@ class ThumbnailLoader {
         fillMode === ThumbnailLoader.FillMode.OVER_FILL);
 
     return new Promise((resolve, reject) => {
-      // Load by using ImageLoaderClient.
+      let requestUrl = this.thumbnailUrl_;
+
+      if (fillMode === ThumbnailLoader.FillMode.OVER_FILL) {
+        // Use the croppedThumbnailUrl_ if available.
+        requestUrl = this.croppedThumbnailUrl_ || this.thumbnailUrl_;
+      }
+
+      if (!requestUrl) {
+        const error = LoadImageResponseStatus.ERROR;
+        reject(new LoadImageResponse(error, 0));
+        return;
+      }
+
       const modificationTime = this.metadata_ && this.metadata_.filesystem &&
           this.metadata_.filesystem.modificationTime &&
           this.metadata_.filesystem.modificationTime.getTime();
-      let request = LoadImageRequest.createRequest({
-        url: this.thumbnailUrl_,
+
+      // Load using ImageLoaderClient.
+      const request = LoadImageRequest.createRequest({
+        url: requestUrl,
         maxWidth: ThumbnailLoader.THUMBNAIL_MAX_WIDTH,
         maxHeight: ThumbnailLoader.THUMBNAIL_MAX_HEIGHT,
         cache: true,
@@ -234,10 +248,6 @@ class ThumbnailLoader {
       });
 
       if (fillMode === ThumbnailLoader.FillMode.OVER_FILL) {
-        // Use cropped thumbnail url if available.
-        request.url = this.croppedThumbnailUrl_ ? this.croppedThumbnailUrl_ :
-                                                  this.thumbnailUrl_;
-
         // Set crop option to image loader. Since image of croppedThumbnailUrl_
         // is 360x360 with current implementation, it's no problem to crop it.
         request.width = 360;