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

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

Review-Url: https://codereview.chromium.org/2514813002
Cr-Commit-Position: refs/heads/master@{#436289}
diff --git a/ios/chrome/browser/metrics/BUILD.gn b/ios/chrome/browser/metrics/BUILD.gn
index 4d4e1c6..e9a7abf 100644
--- a/ios/chrome/browser/metrics/BUILD.gn
+++ b/ios/chrome/browser/metrics/BUILD.gn
@@ -3,6 +3,7 @@
 # found in the LICENSE file.
 
 source_set("metrics") {
+  configs += [ "//build/config/compiler:enable_arc" ]
   sources = [
     "field_trial_synchronizer.cc",
     "field_trial_synchronizer.h",