Convert ios/chrome/commong/block_unittest.mm to ARC.

ios/chrome/common/BUILD.gn

 # Copyright 2015 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/ios/ios_sdk.gni")
 
 source_set("common") {
   configs += [ "//build/config/compiler:enable_arc" ]
   sources = [
@@ skipping 12 matching lines @@
     "//components/version_info",
     "//ios/chrome/common/app_group:main_app",
     "//ios/chrome/common/physical_web",
     "//net",
     "//url",
   ]
 
   libs = [ "QuartzCore.framework" ]
 }
 
-source_set("noarc_unit_tests") {
+source_set("unit_tests") {
   testonly = true
   sources = [
     "block_unittest.mm",
-  ]
-  deps = [
-    ":common",
-    "//base",
-    "//testing/gtest",
-  ]
-}
-
-source_set("unit_tests") {
-  configs += [ "//build/config/compiler:enable_arc" ]
-  testonly = true
-  sources = [
     "ns_regular_expression_unittest.mm",
     "string_util_unittest.mm",
     "x_callback_url_unittest.cc",
   ]
   deps = [
     ":common",
     ":noarc_unit_tests",
     "//base",
     "//testing/gtest",
   ]
+  configs += [ "//build/config/compiler:enable_arc" ]
+}
+
+# The block_unittest.mm has been duplicated during the migration of iOS code
+# to ARC (see crbug.com/624363). As this test checks the interaction of C++
+# objects and Objective-C blocks, it is necessary to keep two version of the
+# test compiled with and without ARC. Remove this file when the conversion to
+# ARC is complete.
+source_set("noarc_unit_tests") {
+  visibility = [ ":unit_tests" ]
+  testonly = true
+  sources = [
+    "noarc_block_unittest.mm",
+  ]
+  deps = [
+    ":common",
+    "//base",
+    "//testing/gtest",
+  ]
 }
 
 buildflag_header("ios_app_bundle_id_prefix_header") {
   header = "ios_app_bundle_id_prefix.h"
   flags = [ "IOS_APP_BUNDLE_ID_PREFIX=\"$ios_app_bundle_id_prefix\"" ]
 }