Use the substr API correctly in FullHashToHashPrefix.

components/safe_browsing_db/v4_protocol_manager_util.cc

Index: components/safe_browsing_db/v4_protocol_manager_util.cc
diff --git a/components/safe_browsing_db/v4_protocol_manager_util.cc b/components/safe_browsing_db/v4_protocol_manager_util.cc
index 44aeee983a4353e95c3463aefd0bda26863b6246..bd097a72b58a30e2b6f31d0408d0de0fc8a0e703 100644
--- a/components/safe_browsing_db/v4_protocol_manager_util.cc
+++ b/components/safe_browsing_db/v4_protocol_manager_util.cc
@@ -305,7 +305,7 @@ bool V4ProtocolManagerUtil::FullHashToHashPrefix(const FullHash& full_hash,
   if (full_hash.size() < prefix_size) {
     return false;
   }
-  *hash_prefix = full_hash.substr(prefix_size);
+  *hash_prefix = full_hash.substr(0, prefix_size);
   return true;
 }