Auto merge of #17903 - BorisChiou:stylo/animation/interpolatematrix_to_css, r=birtles

stylo: Implement ToCss for SpecifiedOperation::{InterpolateMatrix|AccumulateMatrix}.

DevTools may serialize an InterpolateMatrix or AccumulateMatrix by
KeyframeEffectReadOnly::GetProperties() (from Gecko side), so we have to implement both.

---
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix [Bug 1381386](https://bugzilla.mozilla.org/show_bug.cgi?id=1381386).
- [X] These changes do not require tests because only Chrome-Only API needs this, and the serialization of InterpolateMatrix and AccumulateMatrix are not in the spec.

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components/style/properties/longhand/box.mako.rs

@@ -979,7 +979,14 @@ ${helpers.predefined_type(
                     dest, "rotate3d({}, {}, {}, {})",
                     Css(x), Css(y), Css(z), Css(theta)),
                 SpecifiedOperation::Perspective(ref length) => write!(dest, "perspective({})", Css(length)),
-                _ => unreachable!(),
+                SpecifiedOperation::InterpolateMatrix { ref from_list, ref to_list, progress } => {
+                    write!(dest, "interpolatematrix({}, {}, {})",
+                           Css(from_list), Css(to_list), Css(progress))
+                },
+                SpecifiedOperation::AccumulateMatrix { ref from_list, ref to_list, count } => {
+                    write!(dest, "accumulatematrix({}, {}, {})",
+                           Css(from_list), Css(to_list), Css(count))
+                }
             }
         }
     }