Hmm, I think we already handle the 404 case.
Have you tried http://xxyzzzq.github.io/sandbox/media-session/full-test-artwork.html ?

I did found another issue where the artwork is not reset when metadata is set to null.

I believe this is the same then. See https://beaufortfrancois.github.io/sandbox/media-session/404-artwork.html

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/994960d632298a60bb6974a3b0a0217023eef4f0

commit 994960d632298a60bb6974a3b0a0217023eef4f0
Author: zqzhang <zqzhang@chromium.org>
Date: Thu Jan 05 12:48:00 2017

[MediaNotification] Fix logic for resetting MediaImage

This CL fixes the logic for unsetting MediaImage in the notification in
the following two cases:

* When image download returns 404.
* When setting null metadata.

BUG= 676997 
R=mlamouri@chromium.org

Review-Url: https://codereview.chromium.org/2606363002
Cr-Commit-Position: refs/heads/master@{#441636}

[modify] https://crrev.com/994960d632298a60bb6974a3b0a0217023eef4f0/chrome/android/java/src/org/chromium/chrome/browser/media/ui/MediaImageManager.java
[modify] https://crrev.com/994960d632298a60bb6974a3b0a0217023eef4f0/chrome/android/java/src/org/chromium/chrome/browser/media/ui/MediaSessionTabHelper.java
[modify] https://crrev.com/994960d632298a60bb6974a3b0a0217023eef4f0/chrome/android/java_sources.gni
[add] https://crrev.com/994960d632298a60bb6974a3b0a0217023eef4f0/chrome/android/junit/src/org/chromium/chrome/browser/media/ui/MediaImageManagerTest.java