style: Avoid allocating a unique nsStyleSVG and nsStyleBackground per element.

Should also help with https://bugzilla.mozilla.org/show_bug.cgi?id=1360881

components/style/properties/gecko.mako.rs

@@ -2774,10 +2774,6 @@ fn static_assert() {
         % for member in fill_fields.split():
             max_len = cmp::max(max_len, self.gecko.${image_layers_field}.${member}Count);
         % endfor
-
-        // XXXManishearth Gecko does an optimization here where it only
-        // fills things in if any of the properties have been set
-
         unsafe {
             // While we could do this manually, we'd need to also manually
             // run all the copy constructors, so we just delegate to gecko

components/style/properties/properties.mako.rs

@@ -2044,6 +2044,15 @@ impl<'a, T: 'a> StyleStructRef<'a, T>
         }
     }
 
+    /// Get a mutable reference to the owned struct, or `None` if the struct
+    /// hasn't been mutated.
+    pub fn get_if_mutated(&mut self) -> Option<<&mut T> {
+        match *self {
+            StyleStructRef::Owned(ref mut v) => Some(v),
+            StyleStructRef::Borrowed(..) => None,
+        }
+    }
+
     /// Returns an `Arc` to the internal struct, constructing one if
     /// appropriate.
     pub fn build(self) -> Arc<T> {
@@ -2141,6 +2150,13 @@ impl<'a> StyleBuilder<'a> {
         pub fn mutate_${style_struct.name_lower}(&mut self) -> &mut style_structs::${style_struct.name} {
             self.${style_struct.ident}.mutate()
         }
+
+        /// Gets a mutable view of the current `${style_struct.name}` style,
+        /// only if it's been mutated before.
+        pub fn get_${style_struct.name_lower}_if_mutated(&mut self)
+                                                         -> Option<<&mut style_structs::${style_struct.name}> {
+            self.${style_struct.ident}.get_if_mutated()
+        }
     % endfor
 
     /// Returns whether this computed style represents a floated object.
@@ -2546,11 +2562,13 @@ pub fn apply_declarations<'a, F, I>(device: &Device,
     }
 
     % if product == "gecko":
-        // FIXME(emilio): This is effectively creating a new nsStyleBackground
-        // and nsStyleSVG per element. We should only do this when necessary
-        // using the `seen` bitfield!
-        style.mutate_background().fill_arrays();
-        style.mutate_svg().fill_arrays();
+        if let Some(ref mut bg) = style.get_background_if_mutated() {
+            bg.fill_arrays();
+        }
+
+        if let Some(ref mut svg) = style.get_svg_if_mutated() {
+            svg.fill_arrays();
+        }
     % endif
 
     if is_root_element {