CCA: Optimize thumbnail generation for long video

Bug: 1058325
Test: Record a 2hrs video and check the thumbnail generation time
and content manually.

Change-Id: I189e7bdcf3b78c8aa41680456dcd556495e681dd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2179863
Commit-Queue: Shik Chen <shik@chromium.org>
Reviewed-by: Wei Lee <wtlee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#765375}

chrome/browser/resources/chromeos/camera/src/js/gallerybutton.js

@@ -16,6 +16,15 @@ import * as util from './util.js';
  */
 const THUMBNAIL_WIDTH = 240;
 
+/**
+ * The maximum size of video used to generate the thumbnail. The file would be
+ * truncated to that size before generating the thumbnail, which speeds up the
+ * generation process significantly when the file is large. 32MB should be more
+ * than enough for getting the first frame from a video.
+ * @type {number}
+ */
+const VIDEO_THUMBNAIL_SIZE_LIMIT = 32 << 20;
+
 /**
  * Cover photo of gallery button.
  */
@@ -59,8 +68,9 @@ class CoverPhoto {
    * @return {!Promise<!CoverPhoto>}
    */
   static async create(file) {
-    const fileUrl = await filesystem.pictureURL(file);
     const isVideo = filesystem.hasVideoPrefix(file);
+    const limit = isVideo ? VIDEO_THUMBNAIL_SIZE_LIMIT : Infinity;
+    const fileUrl = await filesystem.pictureURL(file, limit);
     const thumbnail =
         await util.scalePicture(fileUrl, isVideo, THUMBNAIL_WIDTH);
     URL.revokeObjectURL(fileUrl);

chrome/browser/resources/chromeos/camera/src/js/models/filesystem.js

@@ -498,14 +498,18 @@ export function getEntries() {
 /**
  * Returns an URL for a picture.
  * @param {!FileEntry} entry File entry.
+ * @param {number} limit Size limit. The file would be truncated if it's larger
+ *     than limit.
  * @return {!Promise<string>} Promise for the result.
  */
-export function pictureURL(entry) {
+export function pictureURL(entry, limit) {
   return new Promise((resolve) => {
-    if (externalDir) {
-      entry.file((file) => resolve(URL.createObjectURL(file)));
-    } else {
-      resolve(entry.toURL());
-    }
+    entry.file((file) => {
+      if (file.size > limit) {
+        file = file.slice(0, limit);
+      }
+      const url = URL.createObjectURL(file);
+      resolve(url);
+    });
   });
 }