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

ios/chrome/common/BUILD.gn

Index: ios/chrome/common/BUILD.gn
diff --git a/ios/chrome/common/BUILD.gn b/ios/chrome/common/BUILD.gn
index 49a2cf1ee826b286360c8f6ff36d4d18156a4d3f..d0b2a470e2cf8eedc16182c7a0d924d3cad967f4 100644
--- a/ios/chrome/common/BUILD.gn
+++ b/ios/chrome/common/BUILD.gn
@@ -30,29 +30,36 @@ source_set("common") {
   libs = [ "QuartzCore.framework" ]
 }
 
-source_set("noarc_unit_tests") {
+source_set("unit_tests") {
   testonly = true
   sources = [
     "block_unittest.mm",
+    "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" ]
 }
 
-source_set("unit_tests") {
-  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 = [
-    "ns_regular_expression_unittest.mm",
-    "string_util_unittest.mm",
-    "x_callback_url_unittest.cc",
+    "noarc_block_unittest.mm",
   ]
   deps = [
     ":common",
-    ":noarc_unit_tests",
     "//base",
     "//testing/gtest",
   ]