Destroy store on task runner. That's where all store operations happen.

components/safe_browsing_db/v4_database.cc

Index: components/safe_browsing_db/v4_database.cc
diff --git a/components/safe_browsing_db/v4_database.cc b/components/safe_browsing_db/v4_database.cc
index 30a3b0f538d5756e5314329a88cdfb9a5f0191dd..dc64e42f829d636133610ca557095cc47429d997 100644
--- a/components/safe_browsing_db/v4_database.cc
+++ b/components/safe_browsing_db/v4_database.cc
@@ -142,6 +142,7 @@ void V4Database::UpdatedStoreReady(ListIdentifier identifier,
   DCHECK_CURRENTLY_ON(BrowserThread::IO);
   DCHECK(pending_store_updates_);
   if (new_store) {
+    V4Store::Destroy(std::move((*store_map_)[identifier]));
     (*store_map_)[identifier] = std::move(new_store);

[Scott Hess - ex-Googler, 2016/10/18 02:57:01]

Does this work?

I mean, I think it probably does, but I'm having troubles describing how.  I'll
just throw this out there, but leave it in your hands whether you want to go
there:

std::swap((*store_map_)[identifier], new_store);
if (new_store) {
  V4Store::Destroy(std::move(new_store));
}

Of course, abusing new_store as temp storage like this is optional, and I
haven't read Destroy() yet to see if that's where the null handling should be.

[Scott Hess - ex-Googler, 2016/10/18 02:57:55]

Or, of course:

(*store_map_)[identifier].swap(new_store);

   }