[ObjC ARC] Converts ios/chrome/browser/metrics:unit_tests_internal to ARC.

Automatically generated ARCMigrate commit
Notable issues:None
BUG=624363
TEST=None

Review-Url: https://codereview.chromium.org/2894653002
Cr-Commit-Position: refs/heads/master@{#475331}
diff --git a/ios/chrome/browser/metrics/BUILD.gn b/ios/chrome/browser/metrics/BUILD.gn
index 91c6ea4..f9703b0 100644
--- a/ios/chrome/browser/metrics/BUILD.gn
+++ b/ios/chrome/browser/metrics/BUILD.gn
@@ -129,6 +129,7 @@
 }
 
 source_set("unit_tests_internal") {
+  configs += [ "//build/config/compiler:enable_arc" ]
   testonly = true
   sources = [
     "first_user_action_recorder_unittest.cc",