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

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

Review-Url: https://codereview.chromium.org/2513993002
Cr-Commit-Position: refs/heads/master@{#436286}

ios/chrome/browser/metrics/BUILD.gn

@@ -57,6 +57,7 @@ source_set("metrics") {
 }
 
 source_set("unit_tests") {
+  configs += [ "//build/config/compiler:enable_arc" ]
   testonly = true
   sources = [
     "ios_chrome_metrics_service_accessor_unittest.cc",

ios/chrome/browser/metrics/mobile_session_shutdown_metrics_provider_unittest.mm

@@ -20,6 +20,10 @@
 #include "testing/gtest/include/gtest/gtest-param-test.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 // An MobileSessionShutdownMetricsProvider that returns fake values for the last
 // session environment query methods.
 class MobileSessionShutdownMetricsProviderForTesting

ios/chrome/browser/metrics/previous_session_info_unittest.mm

@@ -10,6 +10,10 @@
 #include "testing/gtest/include/gtest/gtest.h"
 #include "testing/gtest_mac.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 namespace {
 
 // Key in the UserDefaults for a boolean value keeping track of memory warnings.