[ObjC ARC] Converts ios/chrome/common:unit_tests 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 d71c521771d56935a8fa6b2b00f1bd7af0ca2142..b21a390db425a6248fcc2391ff800ed2b422e597 100644
--- a/ios/chrome/common/BUILD.gn
+++ b/ios/chrome/common/BUILD.gn
@@ -29,10 +29,22 @@ source_set("common") {
   libs = [ "QuartzCore.framework" ]
 }
 
-source_set("unit_tests") {
+source_set("noarc_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",