[Autofill] Handle unstable refill identifiers in TriggerRefill().

FormStructure::GetIdentifierForRefill() is not stable across dynamic
changes. If a cache update happens before the refill is executed,
the identifier may change in the meantime and we may thus refer
to the wrong form.

Bug: 1114787
Change-Id: I6792829192414e8e52d239b62899ea45e0465a3a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2404162Reviewed-by: Dominic Battré <battre@chromium.org>
Reviewed-by: Matthias Körber <koerber@google.com>
Commit-Queue: Christoph Schwering <schwering@google.com>
Cr-Commit-Position: refs/heads/master@{#806148}

components/autofill/core/browser/autofill_manager.cc

@@ -2518,7 +2518,12 @@ void AutofillManager::TriggerRefill(const FormData& form) {
 
   auto itr =
       filling_contexts_map_.find(form_structure->GetIdentifierForRefill());
-  DCHECK(itr != filling_contexts_map_.end());
+
+  // Since GetIdentifierForRefill() is not stable across dynamic changes,
+  // |filling_contexts_map_| may not contain the element anymore.
+  if (itr == filling_contexts_map_.end())
+    return;
+
   FillingContext* filling_context = itr->second.get();
 
   // The refill attempt can happen from different paths, some of which happen