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

ios/chrome/browser/ui/ui_util_unittest.mm

Index: ios/chrome/browser/ui/ui_util_unittest.mm
diff --git a/ios/chrome/browser/ui/ui_util_unittest.mm b/ios/chrome/browser/ui/ui_util_unittest.mm
index a7b8e8e5b7f813e0f08da05198e6b9843c32c97c..1a2c2d390b4612ac1822e19d106896ef23107eda 100644
--- a/ios/chrome/browser/ui/ui_util_unittest.mm
+++ b/ios/chrome/browser/ui/ui_util_unittest.mm
@@ -11,6 +11,10 @@
 #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(UIUtilTest, AlignToPixel) {
   CGFloat scale = [[UIScreen mainScreen] scale];
   // Pick a few interesting values: already aligned, aligned on retina, and