style: Use auto as the initial value for view-timeline-inset

Per the proposal in https://github.com/w3c/csswg-drafts/issues/7747,
we change view-timeline-inset to have an initial value of auto.

Differential Revision: https://phabricator.services.mozilla.com/D173487

components/style/properties/longhands/ui.mako.rs

@@ -386,7 +386,7 @@ ${helpers.predefined_type(
 ${helpers.predefined_type(
     "view-timeline-inset",
     "ViewTimelineInset",
-    "computed::ViewTimelineInset::zero()",
+    "computed::ViewTimelineInset::default()",
     vector=True,
     need_index=True,
     engines="gecko",

components/style/values/computed/ui.rs

@@ -23,16 +23,3 @@ pub type ScrollbarColor = generics::GenericScrollbarColor<Color>;
 
 /// A computed value for the `view-timeline-inset` property.
 pub type ViewTimelineInset = generics::GenericViewTimelineInset<LengthPercentage>;
-
-impl ViewTimelineInset {
-    /// Returns the initial value, `0`.
-    #[inline]
-    pub fn zero() -> Self {
-        use crate::Zero;
-
-        Self {
-            start: Zero::zero(),
-            end: Zero::zero(),
-        }
-    }
-}

components/style/values/generics/ui.rs

@@ -169,3 +169,12 @@ where
         Ok(())
     }
 }
+
+impl<LengthPercent> Default for ViewTimelineInset<LengthPercent> {
+    fn default() -> Self {
+        Self {
+            start: GenericLengthPercentageOrAuto::auto(),
+            end: GenericLengthPercentageOrAuto::auto(),
+        }
+    }
+}