[ObjC ARC] Converts ios/chrome/common to ARC

ios/chrome/common/string_util.mm

Index: ios/chrome/common/string_util.mm
diff --git a/ios/chrome/common/string_util.mm b/ios/chrome/common/string_util.mm
index 395f66035276d4a244542b05aaa3a9063bc4469f..493a0765d3810140e99d66efbad2e025b726eb06 100644
--- a/ios/chrome/common/string_util.mm
+++ b/ios/chrome/common/string_util.mm
@@ -12,6 +12,10 @@
 #include "base/strings/stringprintf.h"
 #include "base/strings/sys_string_conversions.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 namespace {
 typedef BOOL (^ArrayFilterProcedure)(id object, NSUInteger index, BOOL* stop);
 typedef NSString* (^SubstringExtractionProcedure)(NSUInteger);