CopyOutputResult must have a working release callback when received over mojo

cc/ipc/copy_output_result.mojom

 // Copyright 2017 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 cc.mojom;
 
 import "cc/ipc/texture_mailbox.mojom";
+import "cc/ipc/texture_mailbox_releaser.mojom";
 import "skia/public/interfaces/bitmap.mojom";
 import "ui/gfx/geometry/mojo/geometry.mojom";
 
 // See cc/output/copy_output_result.h.
 struct CopyOutputResult {
   gfx.mojom.Size size;
   skia.mojom.Bitmap? bitmap;
-  cc.mojom.TextureMailbox texture_mailbox;
+  TextureMailbox texture_mailbox;
+  TextureMailboxReleaser? releaser;

[danakj, 2017/02/10 18:32:50]

If this is optional, TextureMailbox should be too, and they should both be
present when one is present. Can you express that in mojo? For that matter
bitmap and texture_mailbox shouldn't both be present either..

[Ken Rockot(use gerrit already), 2017/02/10 18:56:18]

The way to express this would be to group them:

struct TextureInfo {
  skia.mojom.Bitmap bitmap;
  TextureMailbox texture_mailbox;
  TextureMailboxReleaser releaser;
};

struct CopyOutputResult {
  gfx.mojom.Size size;
  TextureInfo? texture_info;
};


But then, if there's no bitmap, mailbox, or releaser, is there really a
CopyOutputResult at all? Could we just make none of the fields optional, and
have messages which reply with "CopyOutputResult" instead reply with
"CopyOutputResult?" ?

 };