Undo CSSTransitionDiscrete interpolation test changes

Now that we are planning to add a new opt-in syntax for
CSSTransitionDiscrete, the test changes which I made for
CSSTransitionDiscrete will not eventually stick. Since these tests are
interop23 tracked, the other browsers would like them to be updated
ASAP.

I thought about also disabling the CSSTransitionDiscrete flag, which
would make this patch a lot cleaner, but that would break demos and
stuff people are working on so I decided against it.

This was asked for here:
#39871

CSSTransitionDiscrete opt in syntax:
w3c/csswg-drafts#8857

Change-Id: If28b7e9fc4fc0d6dc851e35007fd7614a9b497a0

css/support/interpolation-testcommon.js

@@ -56,7 +56,7 @@
       target.style.setProperty(property, isNeutralKeyframe(from) ? '' : from);
     },
     nonInterpolationExpectations: function(from, to) {
-      return expectFlip(from, to, 0.5);
+      return expectFlip(from, to, -Infinity);
     },
     notAnimatableExpectations: function(from, to, underlying) {
       return expectFlip(from, to, -Infinity);