CopyOutputResult must have a working release callback when received over mojo

cc/ipc/texture_mailbox_struct_traits.h

 // 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.
 
 #ifndef CC_IPC_TEXTURE_MAILBOX_STRUCT_TRAITS_H_
 #define CC_IPC_TEXTURE_MAILBOX_STRUCT_TRAITS_H_
 
 #include "cc/ipc/texture_mailbox.mojom-shared.h"
 #include "cc/resources/texture_mailbox.h"
 #include "gpu/ipc/common/mailbox_holder_struct_traits.h"
 #include "ui/gfx/geometry/mojo/geometry_struct_traits.h"
 #include "ui/gfx/ipc/color/gfx_param_traits.h"
 
 namespace mojo {
 
 template <>
 struct StructTraits<cc::mojom::TextureMailboxDataView, cc::TextureMailbox> {
+  static bool IsNull(const cc::TextureMailbox& input) {
+    return !input.IsTexture();

[danakj, 2017/02/14 23:49:34]

Uh.. so. This is where this nullable thing gets really weird. !IsTexture doesnt
mean it's null. Maybe this is kinda right cuz we don't send software ones.. yet,
but it's awkward.. I wish we did not use nullable mojo fields at all.

+  }
+
+  static void SetToNull(cc::TextureMailbox* output) {
+    *output = cc::TextureMailbox();
+  }
+
   static const gpu::MailboxHolder& mailbox_holder(
       const cc::TextureMailbox& input) {
     return input.mailbox_holder_;
   }
 
   static const gfx::Size& size_in_pixels(const cc::TextureMailbox& input) {
     return input.size_in_pixels_;
   }
 
   static bool is_overlay_candidate(const cc::TextureMailbox& input) {
@@ skipping 40 matching lines @@
 
     return data.ReadMailboxHolder(&out->mailbox_holder_) &&
            data.ReadSizeInPixels(&out->size_in_pixels_) &&
            data.ReadColorSpace(&out->color_space_);
   }
 };
 
 }  // namespace mojo
 
 #endif  // CC_IPC_TEXTURE_MAILBOX_STRUCT_TRAITS_H_