layout: Don't crash on floated generated content.

It doesn't construct the float correctly, but at least it doesn't crash
anymore.

Fixes Reddit.

Closes #3287.
Merge pull request #3293 from pcwalton/floated-generated-content-crash

layout: Don't crash on floated generated content.
Implement basic support for display: inline-block.

This still needs a lot of work, but it covers the basic
cases and improves wikipedia while passing all existing tests.

Tweak reftest to deal with linux/travis black background.