Use 4 value syntax for getComputedStyle of background-position and -webkit-mask-position

This patch forces getComputedStyle of background-position to always return
the 4 value syntax (eg. left 10% top 25px) regardless of input format.
Similar behaviour applies to -webkit-mask-position as well.

This format is dictated by the background-position spec:
http://www.w3.org/TR/css3-background/#the-background-position

Existing tests already check this behaviour and have been updated.

BUG=310977

Review URL: https://codereview.chromium.org/38573003

git-svn-id: svn://svn.chromium.org/blink/trunk@161213 bbb929c8-8fbe-4397-9dbb-9b2b20218538