Factor out NetworkID and caching mechanism from n_q_e.{h,cc}

net/nqe/network_quality_store.h

Index: net/nqe/network_quality_store.h
diff --git a/net/nqe/network_quality_store.h b/net/nqe/network_quality_store.h
new file mode 100644
index 0000000000000000000000000000000000000000..8ac78e97a280ff1e61510a138988b42675d8c7cf
--- /dev/null
+++ b/net/nqe/network_quality_store.h
@@ -0,0 +1,67 @@
+// Copyright 2016 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 NET_NQE_NETWORK_QUALITY_STORE_H_
+#define NET_NQE_NETWORK_QUALITY_STORE_H_
+
+#include <map>
+
+#include "base/macros.h"
+#include "base/threading/thread_checker.h"
+#include "net/base/net_export.h"
+#include "net/nqe/cached_network_quality.h"
+#include "net/nqe/network_id.h"
+
+namespace net {
+
+namespace nqe {
+
+namespace internal {
+
+// NetworkQualityStore holds the network qualities of different networks in
+// memory. Entries are stored in LRU order, and older entries may be evicted.
+class NET_EXPORT_PRIVATE NetworkQualityStore {
+ public:
+  NetworkQualityStore();
+  ~NetworkQualityStore();
+
+  // Stores the network quality |cached_network_quality| of network with ID
+  // |network_id|.
+  void Add(const nqe::internal::NetworkID& network_id,
+           const nqe::internal::CachedNetworkQuality& cached_network_quality);
+
+  // Returns true if the network quality estimate was successfully read
+  // for a network with ID |network_id|, and sets |cached_network_quality| to
+  // the estimate read.
+  bool GetById(const nqe::internal::NetworkID& network_id,
+               nqe::internal::CachedNetworkQuality* cached_network_quality);
+
+ private:
+  // Maximum size of the store that holds network quality estimates.
+  // A smaller size may reduce the cache hit rate due to frequent evictions.
+  // A larger size may affect performance.
+  static const size_t kMaximumNetworkQualityCacheSize = 10;
+
+  // This does not use an unordered_map or hash_map for code simplicity (the key
+  // just implements operator<, rather than hash and equality) and because the
+  // map is tiny.
+  typedef std::map<nqe::internal::NetworkID,
+                   nqe::internal::CachedNetworkQuality>
+      CachedNetworkQualities;
+
+  // Data structure that stores the qualities of networks.
+  CachedNetworkQualities cached_network_qualities_;
+
+  base::ThreadChecker thread_checker_;
+
+  DISALLOW_COPY_AND_ASSIGN(NetworkQualityStore);
+};
+
+}  // namespace internal
+
+}  // namespace nqe
+
+}  // namespace net
+
+#endif  // NET_NQE_NETWORK_QUALITY_STORE_H_