[css-writing-modes-3] 3 additional background-position tests

Hello,

http://test.csswg.org/source/css-writing-modes-3/background-position-vrl-018.xht

http://test.csswg.org/source/css-writing-modes-3/background-position-vrl-020.xht

http://test.csswg.org/source/css-writing-modes-3/background-position-vrl-022.xht

Firefox 39.0a1 buildID=20150322155329 and IE11.0.16 pass these 3 tests.

Chrome 43.0.2342.2 dev fails these 3 tests.

Review comments are welcomed.

Gérard
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