Write mojom and StructTraits for DisplaySnapshot.

ui/display/mojo/display_struct_traits_test.mojom

Index: ui/display/mojo/display_struct_traits_test.mojom
diff --git a/ui/display/mojo/display_struct_traits_test.mojom b/ui/display/mojo/display_struct_traits_test.mojom
index d478a1f254d9d9eeecf3f27c596fe9d2668feefe..209dd5437e41ebbd8848a3090171dabaf373ecdc 100644
--- a/ui/display/mojo/display_struct_traits_test.mojom
+++ b/ui/display/mojo/display_struct_traits_test.mojom
@@ -7,6 +7,7 @@ module display.mojom;
 import "ui/display/mojo/display.mojom";
 import "ui/display/mojo/display_layout.mojom";
 import "ui/display/mojo/display_mode.mojom";
+import "ui/display/mojo/display_snapshot_mojo.mojom";
 
 interface DisplayStructTraitsTest {
   [Sync]
@@ -16,6 +17,10 @@ interface DisplayStructTraitsTest {
   EchoDisplayMode(DisplayMode in) => (DisplayMode out);
 
   [Sync]
+  EchoDisplaySnapshotMojo(DisplaySnapshotMojo in) => (DisplaySnapshotMojo
+    out);
+
+  [Sync]
   EchoDisplayPlacement(DisplayPlacement in) => (DisplayPlacement out);
 
   [Sync]