Write mojom and StructTraits for display::GammaRampRGBEntry

ui/display/mojo/display_struct_traits_test.mojom

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 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/gamma_ramp_rgb_entry.mojom";
 
 interface DisplayStructTraitsTest {
   [Sync]
   EchoDisplay(Display in) => (Display out);
 
   [Sync]
   EchoDisplayMode(DisplayMode in) => (DisplayMode out);
 
   [Sync]
   EchoDisplayPlacement(DisplayPlacement in) => (DisplayPlacement out);
 
   [Sync]
   EchoDisplayLayout(DisplayLayout in) => (DisplayLayout out);
+
+  [Sync]
+  EchoGammaRampRGBEntry(GammaRampRGBEntry in) => (GammaRampRGBEntry out);
 };