Fix background-repeat CSSOM serialization

Conversion from background-repeat-x/y longhands to background-repeat
shorthand was not correct for multiple background repeat values.
This patch fixes the behaviour.

TEST=fast/css/background-repeat-serialize.html

BUG=366897

Review URL: https://codereview.chromium.org/291303004

git-svn-id: svn://svn.chromium.org/blink/trunk@174770 bbb929c8-8fbe-4397-9dbb-9b2b20218538