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