Fix getComputedStyle for calc on several properties

This patch fixes getComputedStyle for calc values on vertical-align,
flex-basis and the nonstandard properties background-position-x/y and
-webkit-mask-position-x/y, which currently returns the empty string
and asserts in a debug build. We currently use the wrong function in
CSSComputedStyleDeclaration for these, so I've fixed these and removed
this codepath to prevent us from making the same error in the future.

This also incidentally fixes getComputedStyle on these properties when
the user has zoomed on their browser.

Review URL: https://codereview.chromium.org/294903012

git-svn-id: svn://svn.chromium.org/blink/trunk@174563 bbb929c8-8fbe-4397-9dbb-9b2b20218538