[IndexedDB] Pool and evict leveldb iterators, to save memory

content/browser/indexed_db/leveldb/leveldb_transaction.h

Index: content/browser/indexed_db/leveldb/leveldb_transaction.h
diff --git a/content/browser/indexed_db/leveldb/leveldb_transaction.h b/content/browser/indexed_db/leveldb/leveldb_transaction.h
index a701a94748a6b02c10dcfdbd4f947084fa0f2a8f..3a55121ba254d0f36e5d84e3fda63dab098b2783 100644
--- a/content/browser/indexed_db/leveldb/leveldb_transaction.h
+++ b/content/browser/indexed_db/leveldb/leveldb_transaction.h
@@ -93,6 +93,7 @@ class CONTENT_EXPORT LevelDBTransaction
     leveldb::Status Prev() override;
     base::StringPiece Key() const override;
     base::StringPiece Value() const override;
+
     bool IsDeleted() const;
 
     // Mark the current record as deleted.
@@ -123,6 +124,8 @@ class CONTENT_EXPORT LevelDBTransaction
     leveldb::Status Prev() override;
     base::StringPiece Key() const override;
     base::StringPiece Value() const override;
+    void EvictWorkingMemory() override;
+
     void DataChanged();
 
     // Mark the current record as deleted. If an existing record
@@ -141,12 +144,17 @@ class CONTENT_EXPORT LevelDBTransaction
     bool DataIteratorIsLower() const;
     bool DataIteratorIsHigher() const;
 
+    void LoadDBIteratorIfEvicted();
+
     scoped_refptr<LevelDBTransaction> transaction_;
     const LevelDBComparator* comparator_;
     mutable std::unique_ptr<DataIterator> data_iterator_;
     std::unique_ptr<LevelDBIterator> db_iterator_;
     LevelDBIterator* current_ = nullptr;
 
+    bool is_evicted_ = false;
+    std::string key_after_eviction_;
+
     Direction direction_ = FORWARD;
     mutable bool data_changed_ = false;