Simplify getComputedStyle on background/mask-box properties

This patch simplifies getComputedStyle on background and mask-box
properties so that we always compute a CSSValueList instead of having
single values as a special case. This has some visible difference if
using getPropertyCSSValue, but it's unlikely that this will break any
existing web content (firefox is also returns a list in these cases
and IE doesn't implement the deprecated getPropertyCSSValue).

Review URL: https://codereview.chromium.org/405913002

git-svn-id: svn://svn.chromium.org/blink/trunk@179301 bbb929c8-8fbe-4397-9dbb-9b2b20218538