Implement gfx::SelectionBound StructTraits

ui/gfx/mojo/traits_test_service.mojom

Index: ui/gfx/mojo/traits_test_service.mojom
diff --git a/ui/gfx/mojo/traits_test_service.mojom b/ui/gfx/mojo/traits_test_service.mojom
index 0dd76d96bfcd8a18edbec24d7078d1da068c9832..02afb889e0dd9e1987132316970e85419aa5a258 100644
--- a/ui/gfx/mojo/traits_test_service.mojom
+++ b/ui/gfx/mojo/traits_test_service.mojom
@@ -4,11 +4,15 @@
 
 module gfx.mojom;
 
+import "ui/gfx/mojo/selection_bound.mojom";
 import "ui/gfx/mojo/transform.mojom";
 
 // All functions on this interface echo their arguments to test StructTraits
 // serialization and deserialization.
 interface TraitsTestService {
   [Sync]
+  EchoSelectionBound(SelectionBound s) => (SelectionBound pass);
+
+  [Sync]
   EchoTransform(Transform t) => (Transform pass);
 };