[css-backgrounds] Border width rounding affects collapsed border conflict resolution

[wangxianzhu]

http://jsfiddle.net/zmav431j/

<table style="border-collapse: collapse">
  <tr><td style="border: 5.1px solid blue">BLUE</td></tr>
  <tr><td style="border: 5.9px solid red">RED</td></tr>
</table>

On Firefox (and perhaps WebKit, and perhaps future Blink), the color of the border between the two table cells depends on the device scale factor. When dsf=1, it's blue. When dsf=2, it's red.

This issue depends on #5210.