Move interpolated Length unit types out of InterpolableValue into NonInterpolableValue

LayoutTests/animations/retain-zero-percent.html

Index: LayoutTests/animations/retain-zero-percent.html
diff --git a/LayoutTests/animations/retain-zero-percent.html b/LayoutTests/animations/retain-zero-percent.html
new file mode 100644
index 0000000000000000000000000000000000000000..4838d467efddec35cc9153dca5491c304a75ee8b
--- /dev/null
+++ b/LayoutTests/animations/retain-zero-percent.html
@@ -0,0 +1,11 @@
+<script src="../resources/testharness.js"></script>
+<script src="../resources/testharnessreport.js"></script>
+<div id="target"></div>
+<script>
+test(function() {
+  var animation = target.animate([{motionOffset: '-1%'}, {motionOffset: '1%'}], 1);
+  animation.pause();
+  animation.currentTime = 0.5;
+  assert_equals(getComputedStyle(target).motionOffset, '0%');
+});
+</script>