Fix the crash in the Manual fallback for "Show all passwords".

The AutofillAgent can be null in some cases.

BUG=821502

Change-Id: Iada92ce5889643a3eda1d02cb499a16699e73015
Reviewed-on: https://chromium-review.googlesource.com/997797
Commit-Queue: Vadym Doroshenko <dvadym@chromium.org>
Reviewed-by: Vadym Doroshenko <dvadym@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548766}

components/password_manager/core/browser/password_manager_util.cc

@@ -169,6 +169,8 @@ bool ShowAllSavedPasswordsContextMenuEnabled() {
 
 void UserTriggeredShowAllSavedPasswordsFromContextMenu(
     autofill::AutofillClient* autofill_client) {
+  if (!autofill_client)
+    return;
   autofill_client->ExecuteCommand(
       autofill::POPUP_ITEM_ID_ALL_SAVED_PASSWORDS_ENTRY);
   password_manager::metrics_util::LogContextOfShowAllSavedPasswordsAccepted(