iOS Autofill: Fix accidental Array to String conversion in autofill_controller.js

Ther accidentally converts |controlElementsWithAutocomplete|
to a string before passing it to __gCrWeb.fill.formOrFieldsetsToFormData().

Bug: None
Change-Id: I7d97914d55d96639296bed2af0c8090fee1fb99c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1914798Reviewed-by: Moe Ahmadi <mahmadi@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715512}

components/autofill/ios/browser/resources/autofill_controller.js

@@ -587,7 +587,7 @@ function unownedFormElementsAndFieldSetsToFormData_(
   for (var index = 0; index < controlElements.length; index++) {
     if (controlElements[index].hasAttribute('autocomplete') &&
         controlElements[index].getAttribute('autocomplete') !== 'off') {
-      controlElementsWithAutocomplete += controlElements[index];
+      controlElementsWithAutocomplete.push(controlElements[index]);
     }
   }