Stop rendering text inside flex boxes.

Text can only only inside paragraphs and inlines. This patch makes it
so we stop putting such text nodes in the render tree at all if their
parent is not a paragraph or an inline.

This is the final step in making it so that we don't create anonymous
renderers, which fixes a crash in the new custom layout code.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/1077473002

examples/home.sky

 #!mojo mojo:sky_viewer
 <style>
+ div { display: paragraph; }
  .h1 { font-size: 2em; margin: 1em; }
  .p { margin: 0.5em 1em; color: #bcd8f5; font-weight: 900; }
 </style>