Add test coverage for interpolations using CSS wide keywords and neutral keyframes

LayoutTests/animations/interpolation/scale-interpolation.html

Index: LayoutTests/animations/interpolation/scale-interpolation.html
diff --git a/LayoutTests/animations/interpolation/scale-interpolation.html b/LayoutTests/animations/interpolation/scale-interpolation.html
index 582cbc9a35434e6874a5655d15be017d66a0197c..41344b5127a90ca28158e256cd76f60fd48e7975 100644
--- a/LayoutTests/animations/interpolation/scale-interpolation.html
+++ b/LayoutTests/animations/interpolation/scale-interpolation.html
@@ -5,9 +5,10 @@
   scale: 0.5 1 2
 }
 .target {
-  width: 100px;
-  height: 100px;
+  width: 10px;
+  height: 10px;
   background-color: black;
+  scale: 1.1;
 }
 .expected {
   background-color: green;
@@ -22,6 +23,32 @@
 <script>
 assertInterpolation({
   property: 'scale',
+  from: '',
+  to: '1.5',
+}, [
+  {at: -1, is: '0.7 0.7'},
+  {at: 0, is: '1.1 1.1'},
+  {at: 0.25, is: '1.2 1.2'},
+  {at: 0.75, is: '1.4 1.4'},
+  {at: 1, is: '1.5 1.5'},
+  {at: 2, is: '1.9 1.9'},
+]);
+
+assertInterpolation({
+  property: 'scale',
+  from: 'unset',
+  to: '1.5',
+}, [
+  {at: -1, is: '0.5 0.5'},
+  {at: 0, is: '1'},
+  {at: 0.25, is: '1.125 1.125'},
+  {at: 0.75, is: '1.375 1.375'},
+  {at: 1, is: '1.5 1.5'},
+  {at: 2, is: '2 2'},
+]);
+
+assertInterpolation({
+  property: 'scale',
   from: '-10',
   to: '10',
 }, [