style: Record on a computed::Context whether we are computing a SMIL animated value.

components/style/gecko/media_queries.rs

@@ -642,6 +642,7 @@ impl Expression {
             in_media_query: true,
             // TODO: pass the correct value here.
             quirks_mode: quirks_mode,
+            for_smil_animation: false,
         };
 
         let required_value = match self.value {

components/style/properties/properties.mako.rs

@@ -2996,6 +2996,7 @@ where
         cached_system_font: None,
         in_media_query: false,
         quirks_mode: quirks_mode,
+        for_smil_animation: false,
     };
 
     let ignore_colors = !device.use_document_colors();

components/style/servo/media_queries.rs

@@ -246,6 +246,7 @@ impl Range<specified::Length> {
             in_media_query: true,
             cached_system_font: None,
             quirks_mode: quirks_mode,
+            for_smil_animation: false,
         };
 
         match *self {

components/style/stylesheets/viewport_rule.rs

@@ -709,6 +709,7 @@ impl MaybeNew for ViewportConstraints {
             cached_system_font: None,
             in_media_query: false,
             quirks_mode: quirks_mode,
+            for_smil_animation: false,
         };
 
         // DEVICE-ADAPT § 9.3 Resolving 'extend-to-zoom'

components/style/values/computed/mod.rs

@@ -95,6 +95,12 @@ pub struct Context<'a> {
 
     /// The quirks mode of this context.
     pub quirks_mode: QuirksMode,
+
+    /// Whether this computation is being done for a SMIL animation.
+    ///
+    /// This is used to allow certain properties to generate out-of-range
+    /// values, which SMIL allows.
+    pub for_smil_animation: bool,
 }
 
 impl<'a> Context<'a> {