Implement StructTraits for various cc and gpu types

gpu/ipc/common/mailbox_holder.mojom

Index: gpu/ipc/common/mailbox_holder.mojom
diff --git a/gpu/ipc/common/mailbox_holder.mojom b/gpu/ipc/common/mailbox_holder.mojom
index 773f40256ca7e68da1df733ea6c4c6bd7eaa93d4..eceaff3cf2505bdd9f04f707f0c5132404ae3d6d 100644
--- a/gpu/ipc/common/mailbox_holder.mojom
+++ b/gpu/ipc/common/mailbox_holder.mojom
@@ -4,5 +4,12 @@
 
 module gpu.mojom;
 
-[Native]
-struct MailboxHolder;
+import "gpu/ipc/common/mailbox.mojom";
+import "gpu/ipc/common/sync_token.mojom";
+
+// See gpu/command_buffer/common/mailbox_holder.h.
+struct MailboxHolder {
+  gpu.mojom.Mailbox mailbox;
+  gpu.mojom.SyncToken sync_token;
+  uint32 texture_target;
+};