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

ios/chrome/browser/install_time_util_unittest.mm

Index: ios/chrome/browser/install_time_util_unittest.mm
diff --git a/ios/chrome/browser/install_time_util_unittest.mm b/ios/chrome/browser/install_time_util_unittest.mm
index e5cd15985c0aa437aebd19dcf24b1517e80807c1..e539e5785203111e30f3c0742ffe52e6474bc4af 100644
--- a/ios/chrome/browser/install_time_util_unittest.mm
+++ b/ios/chrome/browser/install_time_util_unittest.mm
@@ -6,6 +6,10 @@
 #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
+
 TEST(InstallTimeUtilTest, ComputeInstallationTime) {
   const base::Time null_time = base::Time();
   const base::Time now = base::Time::Now();