Explicitly call load() on video elements when generating thumbnails.

The image loader used by the Files App relies on setting the preload
attribute on a video element to trigger it loading some content to use
as a thumbnail. In some circumstances (e.g. data saver), preload is
ignored for video elements.

Invoke load() on the video element explicitly to ensure it loads the
video.

Bug: 888962
Change-Id: I2114bbb36feac3eafbb90cae2babd67d93247a25
Reviewed-on: https://chromium-review.googlesource.com/c/1301053Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Commit-Queue: Sam McNally <sammc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#603012}

ui/file_manager/image_loader/request.js

@@ -330,6 +330,7 @@ ImageRequest.prototype.createVideoThumbnailUrl_ = function(url) {
           video.currentTime = ImageRequest.VIDEO_THUMBNAIL_POSITION;
           video.preload = 'auto';
           video.src = url;
+          video.load();
         }),
         new Promise((resolve) => {
           setTimeout(resolve, ImageRequest.MAX_MILLISECONDS_TO_LOAD_VIDEO);