Reland 1: "mac: Add auto-close and share-read-only functionality to SharedMemory."

ipc/ipc_message_utils.cc

Index: ipc/ipc_message_utils.cc
diff --git a/ipc/ipc_message_utils.cc b/ipc/ipc_message_utils.cc
index 278d7beffe3b509cb802841356da0ddc736b93e1..0180d743a26a3d90010c758f3c2debb567c6a82f 100644
--- a/ipc/ipc_message_utils.cc
+++ b/ipc/ipc_message_utils.cc
@@ -563,6 +563,12 @@ void ParamTraits<base::SharedMemoryHandle>::Write(Message* m,
       bool result = p.GetSize(&size);
       DCHECK(result);
       ParamTraits<size_t>::Write(m, size);
+
+      // If the caller intended to pass ownership to the IPC stack, release a
+      // reference.
+      if (p.OwnershipPassesToIPC())
+        p.Close();
+
       break;
   }
 }