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 7185abd1315cebdced10cf4e4873ee8a354db0c4..30c92ff0415dce54a2ed443b55a76ef0525f1b45 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";
 import "ui/display/mojo/gamma_ramp_rgb_entry.mojom";
 
 interface DisplayStructTraitsTest {
@@ -17,6 +18,10 @@ interface DisplayStructTraitsTest {
   EchoDisplayMode(DisplayMode in) => (DisplayMode out);
 
   [Sync]
+  EchoDisplaySnapshotMojo(DisplaySnapshotMojo in) => (DisplaySnapshotMojo
+    out);
+
+  [Sync]
   EchoDisplayPlacement(DisplayPlacement in) => (DisplayPlacement out);
 
   [Sync]