Add general impl in gecko_properties for Transform value and add -moz-window-transform.

components/style/properties/longhand/box.mako.rs

@@ -571,6 +571,7 @@ ${helpers.predefined_type("transform", "Transform",
                           "generics::transform::Transform::none()",
                           extra_prefixes="webkit",
                           animation_value_type="ComputedValue",
+                          gecko_ffi_name="mSpecifiedTransform",
                           flags="CREATES_STACKING_CONTEXT FIXPOS_CB",
                           spec="https://drafts.csswg.org/css-transforms/#propdef-transform")}