Mark optional mojo quads fields as nullable

mojo/services/public/interfaces/surfaces/quads.mojom

 // Copyright 2014 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.
 
 import "mojo/services/public/interfaces/geometry/geometry.mojom"
 import "mojo/services/public/interfaces/surfaces/surface_id.mojom"
 
 module mojo {
 
 struct Color {
@@ skipping 65 matching lines @@
 
   // Allows changing the rect that gets drawn to make it smaller. This value
   // should be clipped to |rect|. This rect lives in content space.
   bool needs_blending;
 
   // Index into the containing pass' shared quad state array which has state
   // (transforms etc) shared by multiple quads.
   int32 shared_quad_state_index;
 
   // Only one of the following will be set, depending on the material.
-  CheckerboardQuadState checkerboard_quad_state;
-  DebugBorderQuadState debug_border_quad_state;
-  IoSurfaceContentQuadState io_surface_quad_state;
-  RenderPassQuadState render_pass_quad_state;
-  SolidColorQuadState solid_color_quad_state;
-  SurfaceQuadState surface_quad_state;
-  TextureQuadState texture_quad_state;
-  TiledContentQuadState tiled_content_quad_state;
-  StreamVideoQuadState stream_video_quad_state;
-  YUVVideoQuadState yuv_video_quad_state;
+  CheckerboardQuadState? checkerboard_quad_state;
+  DebugBorderQuadState? debug_border_quad_state;
+  IoSurfaceContentQuadState? io_surface_quad_state;
+  RenderPassQuadState? render_pass_quad_state;
+  SolidColorQuadState? solid_color_quad_state;
+  SurfaceQuadState? surface_quad_state;
+  TextureQuadState? texture_quad_state;
+  TiledContentQuadState? tiled_content_quad_state;
+  StreamVideoQuadState? stream_video_quad_state;
+  YUVVideoQuadState? yuv_video_quad_state;
 };
 
 enum SkXfermode {
   kClear_Mode = 0,    //!< [0, 0]
   kSrc_Mode,      //!< [Sa, Sc]
   kDst_Mode,      //!< [Da, Dc]
   kSrcOver_Mode,  //!< [Sa + Da - Sa*Da, Rc = Sc + (1 - Sa)*Dc]
   kDstOver_Mode,  //!< [Sa + Da - Sa*Da, Rc = Dc + (1 - Da)*Sc]
   kSrcIn_Mode,    //!< [Sa * Da, Sc * Da]
   kDstIn_Mode,    //!< [Sa * Da, Sa * Dc]
@@ skipping 52 matching lines @@
   int32 id;
   mojo.Rect output_rect;
   mojo.Rect damage_rect;
   mojo.Transform transform_to_root_target;
   bool has_transparent_background;
   Quad[] quads;
   SharedQuadState[] shared_quad_states;
 };
 
 }