Add StructTraits for AcceleratedWidget instances.

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 02afb889e0dd9e1987132316970e85419aa5a258..804e9062ba5413e4b63ffe5ee6a6469f2d585134 100644
--- a/ui/gfx/mojo/traits_test_service.mojom
+++ b/ui/gfx/mojo/traits_test_service.mojom
@@ -4,6 +4,7 @@
 
 module gfx.mojom;
 
+import "ui/gfx/mojo/accelerated_widget.mojom";
 import "ui/gfx/mojo/selection_bound.mojom";
 import "ui/gfx/mojo/transform.mojom";
 
@@ -11,6 +12,9 @@ import "ui/gfx/mojo/transform.mojom";
 // serialization and deserialization.
 interface TraitsTestService {
   [Sync]
+  EchoAcceleratedWidget(AcceleratedWidget w) => (AcceleratedWidget pass);
+
+  [Sync]
   EchoSelectionBound(SelectionBound s) => (SelectionBound pass);
 
   [Sync]