[LevelDBService] Disabling block caching on bulk scans

Bug: 784020
Change-Id: I973eb7cbd7067ce3a95a4ee4781f9d8aa3d54739
Reviewed-on: https://chromium-review.googlesource.com/765096Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Commit-Queue: Daniel Murphy <dmurph@chromium.org>
Cr-Commit-Position: refs/heads/master@{#516102}

components/leveldb/leveldb_database_impl.cc

@@ -27,8 +27,12 @@ template <typename FunctionType>
 leveldb::Status ForEachWithPrefix(leveldb::DB* db,
                                   const leveldb::Slice& key_prefix,
                                   FunctionType function) {
-  std::unique_ptr<leveldb::Iterator> it(
-      db->NewIterator(leveldb::ReadOptions()));
+  leveldb::ReadOptions read_options;
+  // Disable filling the cache for bulk scans. Either this is for deletion
+  // (where caching those blocks doesn't make sense) or a mass-read, which the
+  // user "should" be caching / only needing once.
+  read_options.fill_cache = false;
+  std::unique_ptr<leveldb::Iterator> it(db->NewIterator(read_options));
   it->Seek(key_prefix);
   for (; it->Valid(); it->Next()) {
     if (!it->key().starts_with(key_prefix))