style: Run background-color animations on the compositor.

Changes for nsIDOMWindowUtils.getOMTAValue is in the next commit with come test
cases.

Differential Revision: https://phabricator.services.mozilla.com/D13001

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

@@ -15,7 +15,8 @@ ${helpers.predefined_type(
     animation_value_type="AnimatedColor",
     ignored_when_colors_disabled=True,
     allow_quirks=True,
-    flags="APPLIES_TO_FIRST_LETTER APPLIES_TO_FIRST_LINE APPLIES_TO_PLACEHOLDER",
+    flags="APPLIES_TO_FIRST_LETTER APPLIES_TO_FIRST_LINE APPLIES_TO_PLACEHOLDER \
+           CAN_ANIMATE_ON_COMPOSITOR",
 )}
 
 ${helpers.predefined_type(