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..17a1ebe9d15a71094bd3b5f96a701f39e4963771 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() : texture_target(0), sync_point(0) {}
+
+MailboxHolder::MailboxHolder(const Mailbox& mailbox,
+                             uint32 texture_target,
+                             uint32 sync_point)
+    : mailbox(mailbox),
+      texture_target(texture_target),
+      sync_point(sync_point) {}
+
 }  // namespace gpu