[Autofill Assistant] Added missing include.

Not sure how this one fell through the cracks.

Bug: 806868
Change-Id: Id94a4b8b6550aba450d32175012f4c72d1216de3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1844825
Auto-Submit: Clemens Arbesser <arbesser@google.com>
Reviewed-by: Jordan Demeulenaere <jdemeulenaere@chromium.org>
Commit-Queue: Jordan Demeulenaere <jdemeulenaere@chromium.org>
Cr-Commit-Position: refs/heads/master@{#703631}

chrome/browser/android/autofill_assistant/assistant_collect_user_data_delegate.cc

@@ -8,6 +8,7 @@
 #include <utility>
 
 #include "base/android/jni_string.h"
+#include "base/strings/utf_string_conversions.h"
 #include "chrome/android/features/autofill_assistant/jni_headers/AssistantCollectUserDataNativeDelegate_jni.h"
 #include "chrome/browser/android/autofill_assistant/ui_controller_android.h"
 #include "chrome/browser/autofill/android/personal_data_manager_android.h"