Fix getComputedStyle background-position to output calc values

BUG=260097
Review URL: https://codereview.chromium.org/23651010

Review URL: https://codereview.chromium.org/23651010

git-svn-id: svn://svn.chromium.org/blink/trunk@159351 bbb929c8-8fbe-4397-9dbb-9b2b20218538
3 files changed