layout: Destroy flows properly under `display: none`.

Fixes a crash on http://en.wikipedia.org/wiki/South_China_Sea

It plants a destructor bomb on flows so that this can't happen again.
auto merge of #1540 : pcwalton/servo/destroy-flows, r=larsbergstrom

Fixes a crash on http://en.wikipedia.org/wiki/South_China_Sea

It plants a destructor bomb on flows so that this can't happen again.

r? @larsbergstrom