Test calling getPropertyValue on computed styles for background-position properties.

background-position: 0;
  background-position
    getPropertyValue:    0px 50%
    getPropertyCSSValue: [object CSSValueList]
  background-position-x
    getPropertyValue:    0px
    getPropertyCSSValue: [object CSSPrimitiveValue]
  background-position-y
    getPropertyValue:    50%
    getPropertyCSSValue: [object CSSPrimitiveValue]
background-position: 0 0;
  background-position
    getPropertyValue:    0px 0px
    getPropertyCSSValue: [object CSSValueList]
  background-position-x
    getPropertyValue:    0px
    getPropertyCSSValue: [object CSSPrimitiveValue]
  background-position-y
    getPropertyValue:    0px
    getPropertyCSSValue: [object CSSPrimitiveValue]

background-position: 15px;
  background-position
    getPropertyValue:    15px 50%
    getPropertyCSSValue: [object CSSValueList]
  background-position-x
    getPropertyValue:    15px
    getPropertyCSSValue: [object CSSPrimitiveValue]
  background-position-y
    getPropertyValue:    50%
    getPropertyCSSValue: [object CSSPrimitiveValue]
background-position: 10px 20px;
  background-position
    getPropertyValue:    10px 20px
    getPropertyCSSValue: [object CSSValueList]
  background-position-x
    getPropertyValue:    10px
    getPropertyCSSValue: [object CSSPrimitiveValue]
  background-position-y
    getPropertyValue:    20px
    getPropertyCSSValue: [object CSSPrimitiveValue]

No background-position set
  background-position
    getPropertyValue:    0% 0%
    getPropertyCSSValue: [object CSSValueList]
  background-position-x
    getPropertyValue:    0%
    getPropertyCSSValue: [object CSSPrimitiveValue]
  background-position-y
    getPropertyValue:    0%
    getPropertyCSSValue: [object CSSPrimitiveValue]

