Bug 1556012 [wpt PR 17117] - Add support for timing functions jump-start|end|both|none., a=testonly

Automatic update from web-platform-tests
Add support for timing functions jump-start|end|both|none.

Jump-start is equivalent to start, with a discontinuity at t = 0.
Jump-end is equivalent to end, with a discontinuity at t=1.
Jump-both has discontinuities at t=0 and 1.
Jump-none is continuous at t=0 and t=1.

The patch also addresses some serialization issues with existing
step functions since the code needed to be updated in support of the
new step types.

https://drafts.csswg.org/css-easing-1/#step-easing-functions

https://www.chromestatus.com/feature/5730327525851136

Bug: 827560
Change-Id: I950ed09b2ca72c9e09a4f5e3f8d373cc5acbd07b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1634722
Commit-Queue: Kevin Ellis <kevers@chromium.org>
Reviewed-by: Stephen McGruer <smcgruer@chromium.org>
Reviewed-by: Anders Hartvoll Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/master@{#666871}

--

wp5At-commits: 38bf3be2f0272b35ade3d8c0c91ae49d5efdd013
wpt-pr: 17117

testing/web-platform/tests/css/css-typed-om/the-stylepropertymap/properties/animation-timing-function.html

@@ -19,8 +19,18 @@ runListValuedPropertyTests('animation-timing-function', [
   { syntax: 'ease-in' },
   { syntax: 'ease-out' },
   { syntax: 'ease-in-out' },
-  { syntax: 'step-start' },
-  { syntax: 'step-end' },
+  {
+    syntax: 'step-start',
+    computed: (_, result) => {
+      assert_equals(result.toString(), 'steps(1, start)');
+    }
+  },
+  {
+    syntax: 'step-end',
+    computed: (_, result) => {
+      assert_equals(result.toString(), 'steps(1)');
+    }
+  },
 ]);
 
 runUnsupportedPropertyTests('animation-timing-function', [

testing/web-platform/tests/css/css-typed-om/the-stylepropertymap/properties/transition-timing-function.html

@@ -19,8 +19,18 @@ runListValuedPropertyTests('transition-timing-function', [
   { syntax: 'ease-in' },
   { syntax: 'ease-out' },
   { syntax: 'ease-in-out' },
-  { syntax: 'step-start' },
-  { syntax: 'step-end' },
+  {
+    syntax: 'step-start',
+    computed: (_, result) => {
+      assert_equals(result.toString(), 'steps(1, start)');
+    }
+  },
+  {
+    syntax: 'step-end',
+    computed: (_, result) => {
+      assert_equals(result.toString(), 'steps(1)');
+    }
+  },
 ]);
 
 runUnsupportedPropertyTests('transition-timing-function', [