[iOS] Mark test bundle_data targets as testonly.

ui/base/BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/buildflag_header.gni")
 import("//build/config/compiler/compiler.gni")
 import("//build/config/sanitizers/sanitizers.gni")
 import("//build/config/ui.gni")
 import("//testing/test.gni")
 import("//ui/base/ui_features.gni")
@@ skipping 685 matching lines @@
       "../android/java/src/org/chromium/ui/base/LocalizationUtils.java",
       "../android/java/src/org/chromium/ui/base/ResourceBundle.java",
       "../android/java/src/org/chromium/ui/base/SelectFileDialog.java",
       "../android/java/src/org/chromium/ui/base/TouchDevice.java",
     ]
     jni_package = "base"
   }
 }
 
 bundle_data("ui_base_unittests_bundle_data") {
+  testonly = true
   sources = [
     "test/data/data_pack_unittest/truncated-header.pak",
   ]
   outputs = [
     "{{bundle_resources_dir}}/" +
         "{{source_root_relative_dir}}/{{source_file_part}}",
   ]
 }
 
 # GYP version: ui/base/ui_base_tests.gyp:ui_base_unittests
@@ skipping 221 matching lines @@
   }
   if (is_mac) {
     data += [ "$root_out_dir/ui_unittests Framework.framework/" ]
 
     if (is_asan && symbol_level == 0) {
       data += [ "$root_out_dir/ui_base_unittests.dSYM/" ]
     }
   }
 }
 # TODO(GYP) Mac (ui_base_tests_bundle)