Add interfaces for most major Blimp net components.

blimp/net/blimp_message_checkpoint_observer.h

Index: blimp/net/blimp_message_checkpoint_observer.h
diff --git a/blimp/net/blimp_message_checkpoint_observer.h b/blimp/net/blimp_message_checkpoint_observer.h
new file mode 100644
index 0000000000000000000000000000000000000000..127ce33bc78e0e39bd53256de30431fdacb706c4
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+++ b/blimp/net/blimp_message_checkpoint_observer.h
@@ -0,0 +1,24 @@
+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef BLIMP_NET_BLIMP_MESSAGE_CHECKPOINT_OBSERVER_H_
+#define BLIMP_NET_BLIMP_MESSAGE_CHECKPOINT_OBSERVER_H_
+
+#include "base/basictypes.h"
+
+namespace blimp {
+
+// Allows objects to subscribe to message acknowledgement checkpoints.
+class BlimpMessageCheckpointObserver {
+ public:
+  virtual ~BlimpMessageCheckpointObserver() {}
+
+  // Invoked when the remote end has positively acknowledged the receipt of all
+  // messages with ID <= |message_id|.
+  virtual void OnMessageCheckpoint(int64 message_id) = 0;
+};
+
+}  // namespace blimp
+
+#endif  // BLIMP_NET_BLIMP_MESSAGE_CHECKPOINT_OBSERVER_H_