regression: background images painted only after reflow

[paulrouget]

The follow page only turns red after a reflow (resizing the window for example):

<style>
  body {
    width: 100px;
    height: 100px;
    background-image: url('data:image/gif;base64,R0lGODlhAQABAPAAAP8REf///yH5BAAAAAAALAAAAAABAAEAAAICRAEAOw==');
  }
</style>

It's a regression, and I believe it might be because of #10021. See #10021 (comment).