zoomAdjustedPixelValueForLength should return CSSPrimitiveValue.

None of the functions inside return anything other then CSSPrimitiveValue and
it means it can be used with Pair; IE
  Pair::create(
    zoomAdjustedPixelValueForLength(style->objectPosition().x(), style.get()),
    zoomAdjustedPixelValueForLength(style->objectPosition().y(), style.get()),
    Pair::KeepIdenticalValues));

BUG=

Review URL: https://codereview.chromium.org/26451002

git-svn-id: svn://svn.chromium.org/blink/trunk@159144 bbb929c8-8fbe-4397-9dbb-9b2b20218538