style: Fix timing-function overriding from the keyframe declaration list.

The previous behavior is plain wrong, since that array has always at least one
element, so we effectively couldn't specify anything else than "ease" in our
animations.

components/style/keyframes.rs

@@ -114,15 +114,33 @@ pub struct KeyframesStep {
     /// Declarations that will determine the final style during the step, or
     /// `ComputedValues` if this is an autogenerated step.
     pub value: KeyframesStepValue,
+    /// Wether a animation-timing-function declaration exists in the list of
+    /// declarations.
+    ///
+    /// This is used to know when to override the keyframe animation style.
+    pub declared_timing_function: bool,
 }
 
 impl KeyframesStep {
     #[inline]
     fn new(percentage: KeyframePercentage,
            value: KeyframesStepValue) -> Self {
+        let declared_timing_function = match value {
+            KeyframesStepValue::Declarations(ref declarations) => {
+                declarations.iter().any(|prop_decl| {
+                    match *prop_decl {
+                        PropertyDeclaration::AnimationTimingFunction(..) => true,
+                        _ => false,
+                    }
+                })
+            }
+            _ => false,
+        };
+
         KeyframesStep {
             start_percentage: percentage,
             value: value,
+            declared_timing_function: declared_timing_function,
         }
     }
 }