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

This reverts patch 161213.
http://src.chromium.org/viewvc/blink?revision=161213&view=revision

This patch broke the JQuery Spritely plugin.
Reverting change to avoid breaking websites.

> 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

BUG=328171
R=steveblock@chromium.org

Review URL: https://codereview.chromium.org/100173008

git-svn-id: svn://svn.chromium.org/blink/trunk@163842 bbb929c8-8fbe-4397-9dbb-9b2b20218538