Respect fallback colors when interpolating text-decoration-color.

LayoutTests/animations/interpolation/text-decoration-color-interpolation.html

Index: LayoutTests/animations/interpolation/text-decoration-color-interpolation.html
diff --git a/LayoutTests/animations/interpolation/text-decoration-color-interpolation.html b/LayoutTests/animations/interpolation/text-decoration-color-interpolation.html
index d7c01788239357b108c674e2a04873e3934b1b95..53efa528f0188005aeed311d2c9cf76df0fb3b3d 100644
--- a/LayoutTests/animations/interpolation/text-decoration-color-interpolation.html
+++ b/LayoutTests/animations/interpolation/text-decoration-color-interpolation.html
@@ -4,6 +4,7 @@
 .target {
   color: black;
   text-decoration-line: underline;
+  -webkit-text-fill-color: steelblue;
 }
 .replica {
   color: green;
@@ -28,5 +29,19 @@ assertInterpolation({
   {at: 1, is: 'blue'}, // 0000ff
   {at: 1.5, is: 'blue'}
 ]);
+
+assertInterpolation({
+  property: 'text-decoration-color',
+  from: 'initial',
+  to: 'red'
+}, [
+  {at: -5, is: '#00ffff'},
+  {at: -0.4, is: '#00b6fc'},
+  {at: 0, is: 'steelblue'}, // 4682b4
+  {at: 0.2, is: '#6B6890'},
+  {at: 0.6, is: '#b53448'},
+  {at: 1, is: 'red'}, // ff0000
+  {at: 1.5, is: 'red'}
+]);
 </script>
 </body>