Handle interpolation of SVG <paint> 'currentColor' w/ Web/CSS Animations

LayoutTests/animations/interpolation/svg-fill-interpolation.html

Index: LayoutTests/animations/interpolation/svg-fill-interpolation.html
diff --git a/LayoutTests/animations/interpolation/svg-fill-interpolation.html b/LayoutTests/animations/interpolation/svg-fill-interpolation.html
index d73f7159aafcacc4ac1755d7556e16415d2a839a..9030c1ac48f45b9cd4b4f38e160d2617698318a0 100644
--- a/LayoutTests/animations/interpolation/svg-fill-interpolation.html
+++ b/LayoutTests/animations/interpolation/svg-fill-interpolation.html
@@ -6,6 +6,7 @@
   height: 100px;
   stroke: black;
   stroke-width: 20px;
+  color: blue;
 }
 .replica {
   stroke: green;
@@ -40,6 +41,26 @@ assertInterpolation({
 assertInterpolation({
   property: 'fill',
   from: 'orange',
+  to: 'currentColor'
+}, [
+  {at: 0, is: 'orange'},
+  {at: 0.2, is: '#cc8433'},
+  {at: 0.6, is: '#664299'},
+  {at: 1, is: 'blue'},
+]);
+assertInterpolation({
+  property: 'fill',
+  from: 'currentColor',
+  to: 'orange'
+}, [
+  {at: 0, is: 'blue'},
+  {at: 0.2, is: '#3321cc'},
+  {at: 0.6, is: '#996366'},
+  {at: 1, is: 'orange'},
+]);
+assertInterpolation({
+  property: 'fill',
+  from: 'orange',
   to: 'url(#gradient)'
 }, [
   {at: 0, is: 'orange'},