Add gpu::MailboxHolder to hold state for a gpu::Mailbox

gpu/command_buffer/common/mailbox.cc

Index: gpu/command_buffer/common/mailbox.cc
diff --git a/gpu/command_buffer/common/mailbox.cc b/gpu/command_buffer/common/mailbox.cc
index 8d8393fa63c0a647a992c98d6619156e01e75a92..3d32ebb80b075e6c2a1f6cc9bd1e5bdd2f8922bd 100644
--- a/gpu/command_buffer/common/mailbox.cc
+++ b/gpu/command_buffer/common/mailbox.cc
@@ -31,4 +31,13 @@ void Mailbox::SetName(const int8* n) {
   memcpy(name, n, sizeof(name));
 }
 
+MailboxHolder::MailboxHolder() : target_(0), sync_point_(0) {}
+
+MailboxHolder::MailboxHolder(const Mailbox& mailbox,
+                             uint32 target,
+                             uint32 sync_point)
+    : target_(target), sync_point_(sync_point) {
+  mailbox_.SetName(mailbox.name);
+}
+
 }  // namespace gpu