Add a mojom/StructTrait for display::Display.

ui/display/mojo/display_struct_traits_test.mojom

Index: ui/display/mojo/display_struct_traits_test.mojom
diff --git a/third_party/WebKit/public/platform/mime_registry.mojom b/ui/display/mojo/display_struct_traits_test.mojom
similarity index 55%
copy from third_party/WebKit/public/platform/mime_registry.mojom
copy to ui/display/mojo/display_struct_traits_test.mojom
index dbd258295723c3b23462f09c934c3cc35797aa58..1c20bf2ccf40c2e136e308f8a933c1be5346c01d 100644
--- a/third_party/WebKit/public/platform/mime_registry.mojom
+++ b/ui/display/mojo/display_struct_traits_test.mojom
@@ -2,9 +2,12 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-module blink.mojom;
+module display.mojom;
 
-interface MimeRegistry {
+import "ui/display/mojo/display.mojom";
+
+interface DisplayStructTraitsTest {
   [Sync]
-  GetMimeTypeFromExtension(string extension) => (string mime_type);
+  EchoDisplay(Display in) => (Display out);
 };
+