Add a data structure QuicMultipathTransmissionsMap which is not in use now.

net/quic/quic_multipath_transmissions_map.h

Index: net/quic/quic_multipath_transmissions_map.h
diff --git a/net/quic/quic_multipath_transmissions_map.h b/net/quic/quic_multipath_transmissions_map.h
new file mode 100644
index 0000000000000000000000000000000000000000..cc06c1ac9f2f1017e451810399baf2d35f6b4ca5
--- /dev/null
+++ b/net/quic/quic_multipath_transmissions_map.h
@@ -0,0 +1,61 @@
+// A map manages packets which are transmitted across multiple paths.
+// For example, a packet is originally transmitted on path 1 with packet number
+// 1. Then this packet is retransmitted on path 2 with packet number 1. (1, 1)
+// and (2, 1) are inserted into this map. Suppose (2, 1) is detected lost and
+// gets retransmitted on path 2 with packet 2. (2, 2) will not be inserted
+// because this transmission does not "across" path compared to (2, 1).
+
+#ifndef NET_QUIC_QUIC_MULTIPATH_TRANSMISSIONS_MAP_H_
+#define NET_QUIC_QUIC_MULTIPATH_TRANSMISSIONS_MAP_H_
+
+#include <deque>
+
+#include "base/containers/hash_tables.h"
+#include "base/macros.h"
+#include "net/quic/quic_protocol.h"
+
+namespace net_quic {
+
+typedef std::pair<net::QuicPathId, net::QuicPacketNumber>
+    QuicPathIdPacketNumber;
+
+class QuicMultipathTransmissionsMap {
+ public:
+  typedef std::deque<QuicPathIdPacketNumber> MultipathTransmissionsList;
+  typedef base::hash_map<QuicPathIdPacketNumber, MultipathTransmissionsList*>
+      MultipathTransmissionsMap;
+
+  QuicMultipathTransmissionsMap();
+  ~QuicMultipathTransmissionsMap();
+
+  // Called when a packet is retransmitted on a different path. Adds both
+  // |original_path_id_packet_number| (if not exists) and
+  // |path_id_packet_number| to |transmission_map_|.
+  void OnPacketRetransmittedOnDifferentPath(
+      QuicPathIdPacketNumber original_path_id_packet_number,
+      QuicPathIdPacketNumber path_id_packet_number);
+
+  // Returns all multipath transmissions list if |path_id_packet_number| has
+  // been transmitted across multiple paths, nullptr otherwise.
+  const MultipathTransmissionsList* MaybeGetTransmissionsOnOtherPaths(
+      QuicPathIdPacketNumber path_id_packet_number) const;
+
+  // Called after packet |path_id_packet_number| is received.
+  // If |path_id_packet_number| has been transmitted across multiple paths,
+  // clears all multipath transmissions list and removes each transmission from
+  // |transmission_map_|, does nothing otherwise.
+  void OnPacketHandled(QuicPathIdPacketNumber path_id_packet_number);
+
+ private:
+  // Keys of the map are QuicPathIdPacketNumber, and values are pointers to
+  // lists of multipath transmissions of the same packet. For example, if a
+  // packet has been transmitted as (1, 1) and (2, 1), two entries are added
+  // to this map and both values point to the same list: {(1, 1), (2, 1)}.
+  // The MultipathTransmissionsList is owned by the transmission which is
+  // received first (on any path).
+  MultipathTransmissionsMap transmission_map_;
+};
+
+}  // namespace net_quic
+
+#endif  // NET_QUIC_QUIC_MULTIPATH_TRANSMISSIONS_MAP_H_