Fix broken transition for text-decoration-color.

CSSPropertyEquality::propertiesEqual was not implemented for
text-decoration-color.

Note: The test has a high tolerance, because we really only need to test
that the current value is somewhere between the start value and end value.

BUG=351675
TEST=test-decoration-color-transition.html

Review URL: https://codereview.chromium.org/197223002

git-svn-id: svn://svn.chromium.org/blink/trunk@169215 bbb929c8-8fbe-4397-9dbb-9b2b20218538