leveldb: Add DBTracker for exposing databases to Chrome's memory-infra.

third_party/leveldatabase/env_chromium.h

Index: third_party/leveldatabase/env_chromium.h
diff --git a/third_party/leveldatabase/env_chromium.h b/third_party/leveldatabase/env_chromium.h
index d2e5b83111c79974c2b93622dedf2ec8aaf0eb8c..0afac376d2402f273c0e9a167ee2b7854d41990a 100644
--- a/third_party/leveldatabase/env_chromium.h
+++ b/third_party/leveldatabase/env_chromium.h
@@ -6,17 +6,26 @@
 #define THIRD_PARTY_LEVELDATABASE_ENV_CHROMIUM_H_
 
 #include <deque>
+#include <functional>
+#include <memory>
 #include <set>
 #include <string>
 #include <vector>
 
+#include "base/containers/linked_list.h"
 #include "base/files/file.h"
 #include "base/files/file_path.h"
+#include "base/macros.h"
 #include "base/metrics/histogram.h"
 #include "leveldb/env.h"
 #include "port/port_chromium.h"
 #include "util/mutexlock.h"
 
+namespace leveldb {
+class DB;
+struct Options;
+}
+
 namespace leveldb_env {
 
 // These entries map to values in tools/metrics/histograms/histograms.xml. New
@@ -227,6 +236,37 @@ class ChromiumEnv : public leveldb::Env,
   LockTable locks_;
 };
 
+class DBRegistry {
+ public:
+  static leveldb::Status Open(const leveldb::Options& options,
+                              const std::string& name,
+                              leveldb::DB** dbptr);
+
+  static DBRegistry* GetInstance();
+
+  using Visitor = std::function<void(const leveldb::Options& options,
+                                     const std::string& name,
+                                     leveldb::DB* db)>;
+
+  void Visit(const Visitor& visitor);
+
+ private:
+  class DBProxy;
+  class MDP;
+
+  DBRegistry();
+  ~DBRegistry();
+
+  void Register(DBProxy* proxy);
+  void Unregister(DBProxy* proxy);
+
+  base::Lock lock_;
+  base::LinkedList<DBProxy> proxies_;
+  std::unique_ptr<MDP> mdp_;
+
+  DISALLOW_COPY_AND_ASSIGN(DBRegistry);
+};
+
 }  // namespace leveldb_env
 
 #endif  // THIRD_PARTY_LEVELDATABASE_ENV_CHROMIUM_H_