V4LDM: Add check for HashPrefix match and some tests

components/safe_browsing_db/v4_local_database_manager.h

Index: components/safe_browsing_db/v4_local_database_manager.h
diff --git a/components/safe_browsing_db/v4_local_database_manager.h b/components/safe_browsing_db/v4_local_database_manager.h
index 0bc033f0381e6152e830181729f56f87ac8cbbf8..7fa07fbee4ca5dc95e1ca20b5274d8fede38b3e5 100644
--- a/components/safe_browsing_db/v4_local_database_manager.h
+++ b/components/safe_browsing_db/v4_local_database_manager.h
@@ -58,6 +58,12 @@ class V4LocalDatabaseManager : public SafeBrowsingDatabaseManager {
   bool IsCsdWhitelistKillSwitchOn() override;
 
  private:
+  friend class V4LocalDatabaseManagerTest;
+  void SetTaskRunnerForTest(
+      const scoped_refptr<base::SequencedTaskRunner>& task_runner) {
+    task_runner_ = task_runner;
+  }
+
   ~V4LocalDatabaseManager() override;
 
   // The callback called each time the protocol manager downloads updates