CopyOutputResult must have a working release callback when received over mojo

cc/ipc/texture_mailbox_struct_traits.h

Index: cc/ipc/texture_mailbox_struct_traits.h
diff --git a/cc/ipc/texture_mailbox_struct_traits.h b/cc/ipc/texture_mailbox_struct_traits.h
index dc4d88f5627b254aafc4c62b6457aef8f75bf094..d6a7c60a9384dce04c4dd84c50d363332f2ec11c 100644
--- a/cc/ipc/texture_mailbox_struct_traits.h
+++ b/cc/ipc/texture_mailbox_struct_traits.h
@@ -15,6 +15,14 @@ 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_;