Use emplace back for mixed form warning

This CL addresses a leftover review comment from
crrev.com/c/2292868

Bug: 1093103
Change-Id: I8012c60f9a23108763e7da362a551e3c77e6d7a8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2300090
Commit-Queue: Carlos IL <carlosil@chromium.org>
Commit-Queue: Evan Stade <estade@chromium.org>
Auto-Submit: Carlos IL <carlosil@chromium.org>
Reviewed-by: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#788881}

components/autofill/core/browser/autofill_manager.cc

@@ -2580,7 +2580,7 @@ void AutofillManager::GetAvailableSuggestions(
     Suggestion warning_suggestion(
         l10n_util::GetStringUTF16(IDS_AUTOFILL_WARNING_MIXED_FORM));
     warning_suggestion.frontend_id = POPUP_ITEM_ID_MIXED_FORM_MESSAGE;
-    suggestions->assign(1, warning_suggestion);
+    suggestions->emplace_back(warning_suggestion);
     return;
   }