Change Tbody Border Width

Changing a collapsed border's width should change the width of the table and the width of a shrink-to-fit containing block.

There should be a square with a 4px inner blue border and a 1px outer black border.

Firefox 50.0a1 (nightly 2016-07-07) fails this, so it should probably be upstreamed, except it uses chrome-specific runAfterLayoutAndPaint.

The table cell has rowspan=2 because we almost didn't cover the case where a cell's rowspan exceeded the declared rows.

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