Move Epoch to gfx_traits

Resolves #9222. Moved Epoch from components/msg/compositor_msg to
components/gfx_traits/lib. Updated use statements to reflect the move.

components/gfx_traits/lib.rs

@@ -125,3 +125,14 @@ pub struct LayerProperties {
     /// Whether this layer scrolls its overflow area.
     pub scrolls_overflow_area: bool,
 }
+
+/// A newtype struct for denoting the age of messages; prevents race conditions.
+#[derive(PartialEq, Eq, Debug, Copy, Clone, PartialOrd, Ord, Deserialize, Serialize)]
+pub struct Epoch(pub u32);
+
+impl Epoch {
+    pub fn next(&mut self) {
+        let Epoch(ref mut u) = *self;
+        *u += 1;
+    }
+}