servo: Merge #18788 - Use defualt Debug trait for AnimationValue (from BorisChiou:style/animation_value/debug); r=emilio

We use AnimationValue for animation backend to do interpolation,
accumulation, or computing distance. While debugging it, dumping the
property name is not enough. We need to dump the detailed value contained
in it.

For example:
if we animate ```translate(100px)``` to ```translate(200px)```,
and want to dump ```{:?} => {:?}```, the result is ```transform => transform```.
I think what we want is something like:
```Transform([Translate(100px, 0px, 0px)]) => Transform([Translate(200px, 0px, 0px)])```.

Using default Debug trait is not perfect because there are some redundant type strings,
but it is still better than nothing.

---
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [x] These changes are related to PR #18415.
- [X] These changes do not require tests because this is debug only.

Source-Repo: https://github.com/servo/servo
Source-Revision: 64aadee62c1e0f100ad79ab50a56cd1f84285eeb

--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : 4418d524a31dd01cc7c36fe7254ea3b05e736494

servo/components/style/properties/helpers/animated_properties.mako.rs

@@ -337,7 +337,7 @@ unsafe impl HasSimpleFFI for AnimationValueMap {}
 ///
 /// FIXME: We need to add a path for custom properties, but that's trivial after
 /// this (is a similar path to that of PropertyDeclaration).
-#[derive(Clone, PartialEq)]
+#[derive(Clone, Debug, PartialEq)]
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
 pub enum AnimationValue {
     % for prop in data.longhands:
@@ -352,12 +352,6 @@ pub enum AnimationValue {
     % endfor
 }
 
-impl fmt::Debug for AnimationValue {
-    fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
-        formatter.write_str(self.id().name())
-    }
-}
-
 impl AnimationValue {
     /// Returns the longhand id this animated value corresponds to.
     pub fn id(&self) -> LonghandId {