Add additional information to FAIL lines for animation interpolation tests

This change adds to the FAIL line the parsed value of the expected style, which
can help with debugging. For example ...

FAIL: border-image-width from [0] to [20] was [0] at -0.3, expected [0]

becomes ...

FAIL: border-image-width from [0] to [20] was [0] at -0.3, expected [0] (parsed as [0px])

R=dstockwell@chromium.org

Review URL: https://codereview.chromium.org/54293006

git-svn-id: svn://svn.chromium.org/blink/trunk@161327 bbb929c8-8fbe-4397-9dbb-9b2b20218538