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

LayoutTests/animations/interpolation/z-index-interpolation.html

Index: LayoutTests/animations/interpolation/z-index-interpolation.html
diff --git a/LayoutTests/animations/interpolation/z-index-interpolation.html b/LayoutTests/animations/interpolation/z-index-interpolation.html
index 3f9fe9721cf9d25170441b846be52144c6494b53..bc87d8543791b287a22c12bb3bbdf3ad71f2bf16 100644
--- a/LayoutTests/animations/interpolation/z-index-interpolation.html
+++ b/LayoutTests/animations/interpolation/z-index-interpolation.html
@@ -15,10 +15,14 @@ body {
   padding-top: 5px;
   border: 1px solid;
 }
+.parent {
+  z-index: 15;
+}
 .target {
   position: relative;
   width: 350px;
   height: 10px;
+  z-index: -2;
 }
 .actual {
   background-color: black;
@@ -41,6 +45,45 @@ body {
 
 assertInterpolation({
   property: 'z-index',
+  from: '',
+  to: '5',
+}, [
+  {at: -0.3, is: '-4'},
+  {at: 0, is: '-2'},
+  {at: 0.3, is: '0'},
+  {at: 0.6, is: '2'},
+  {at: 1, is: '5'},
+  {at: 1.5, is: '9'},
+]);
+
+assertNoInterpolation({
+  property: 'z-index',
+  from: 'initial',
+  to: '5',
+});
+
+// We fail to inherit correctly due to style overadjustment: crbug.com/375982
+assertInterpolation({
+  property: 'z-index',
+  from: 'inherit',
+  to: '5',
+}, [
+  {at: -0.3, is: '18'},
+  {at: 0, is: '15'},
+  {at: 0.3, is: '12'},
+  {at: 0.6, is: '9'},
+  {at: 1, is: '5'},
+  {at: 1.5, is: '0'},
+]);
+
+assertNoInterpolation({
+  property: 'z-index',
+  from: 'unset',
+  to: '5',
+});
+
+assertInterpolation({
+  property: 'z-index',
   from: '-5',
   to: '5'
 }, [