Auto merge of #16005 - hiikezoe:animation-compose, r=heycam

Compose animation with servo's hashmap

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This is a PR for https://bugzilla.mozilla.org/show_bug.cgi?id=1340958

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors

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- [X] These changes do not require tests because it's for stylo.

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components/style/properties/declaration_block.rs

@@ -15,6 +15,7 @@ use std::fmt;
 use style_traits::ToCss;
 use stylesheets::Origin;
 use super::*;
+#[cfg(feature = "gecko")] use properties::animated_properties::AnimationValueMap;
 
 /// A declaration [importance][importance].
 ///
@@ -341,6 +342,24 @@ impl PropertyDeclarationBlock {
             }
         }
     }
+
+    /// Convert AnimationValueMap to PropertyDeclarationBlock.
+    #[cfg(feature = "gecko")]
+    pub fn from_animation_value_map(animation_value_map: &AnimationValueMap) -> Self {
+        let mut declarations = vec![];
+        let mut longhands = LonghandIdSet::new();
+
+        for (property, animation_value) in animation_value_map.iter() {
+          longhands.set_transition_property_bit(property);
+          declarations.push((animation_value.uncompute(), Importance::Normal));
+        }
+
+        PropertyDeclarationBlock {
+            declarations: declarations,
+            important_count: 0,
+            longhands: longhands,
+        }
+    }
 }
 
 impl ToCss for PropertyDeclarationBlock {

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

@@ -25,6 +25,7 @@ use properties::longhands::visibility::computed_value::T as Visibility;
 use properties::longhands::z_index::computed_value::T as ZIndex;
 #[cfg(feature = "gecko")] use properties::{PropertyDeclarationId, LonghandId};
 use std::cmp;
+#[cfg(feature = "gecko")] use std::collections::HashMap;
 use std::fmt;
 use style_traits::ToCss;
 use super::ComputedValues;
@@ -252,6 +253,12 @@ impl AnimatedProperty {
     }
 }
 
+/// A collection of AnimationValue that were composed on an element.
+/// This HashMap stores the values that are the last AnimationValue to be
+/// composed for each TransitionProperty.
+#[cfg(feature = "gecko")]
+pub type AnimationValueMap = HashMap<TransitionProperty, AnimationValue>;
+
 /// An enum to represent a single computed value belonging to an animated
 /// property in order to be interpolated with another one. When interpolating,
 /// both values need to belong to the same property.