Fix crash in PasswordFormManager::SavePendingToStore.

PasswordFormManager::CreatePendingCredentials is complicated and it's possible
that we want to update a credential but can't find it by username in |best_matches|.
The CL changes the DCHECK into an 'if'. At this point it doesn't make sense to invest
into this class more time.

Bug: 961252
Change-Id: I61ea449cb8b035c72faae7268eb377fc687f7d8e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1602638Reviewed-by: Dominic Battré <battre@chromium.org>
Reviewed-by: Vadym Doroshenko <dvadym@chromium.org>
Commit-Queue: Dominic Battré <battre@chromium.org>
Cr-Commit-Position: refs/heads/master@{#658704}

components/password_manager/core/browser/password_form_manager.cc

@@ -1049,8 +1049,8 @@ void PasswordFormManager::SavePendingToStore(bool update) {
     if (!pending_credentials_.IsFederatedCredential()) {
       auto updated_password_it =
           best_matches_.find(pending_credentials_.username_value);
-      DCHECK(best_matches_.end() != updated_password_it);
-      old_password = updated_password_it->second->password_value;
+      if (updated_password_it != best_matches_.end())
+        old_password = updated_password_it->second->password_value;
     }
 
     form_saver_->Update(pending_credentials_, matches, old_password);