Create a constellation_traits crate (#36088)

This change creates a `constellation_traits` crate. Previously messages
to the `Constellation` were in the `compositing_traits` crate, which
came about organically. This change moves these to a new crate which
also contains data types that are used in both compositing/libservo and
script (ie types that cross the process boundary). The idea is similar
to `embedding_traits`, but this is meant for types not exposed to the
API.

This change allows deduplicating `UntrustedNodeAddress`, which
previously had two versions to avoid circular dependencies.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

components/script/iframe_collection.rs

@@ -6,10 +6,11 @@ use std::cell::Cell;
 use std::default::Default;
 
 use base::id::BrowsingContextId;
+use constellation_traits::{WindowSizeData, WindowSizeType};
 use euclid::{Scale, Size2D};
 use fnv::FnvHashMap;
 use script_layout_interface::IFrameSizes;
-use script_traits::{IFrameSizeMsg, WindowSizeData, WindowSizeType};
+use script_traits::IFrameSizeMsg;
 use style_traits::CSSPixel;
 use webrender_api::units::DevicePixel;