Block formatting contexts should never collapse margins

[pcwalton]

Here is a test case:

<style>

  main {
    height: 300px;
    width: 250px;
    background-image: linear-gradient(to bottom, red, blue);
    overflow: scroll;
  }

  div {
    margin-top: 200px;
    height: 200px;
    width: 200px;
    overflow: scroll;
  }

  p {
    height: 250px;
    background-image: linear-gradient(to bottom, white, black);
    margin: 0;
  }

</style>
<main>
  <div>
    <p></p>
  </div>
</main>

cc @paulrouget