[ObjC ARC] Converts ios/chrome/browser/browser_state to ARC.

ios/chrome/browser/browser_state/chrome_browser_state.mm

Index: ios/chrome/browser/browser_state/chrome_browser_state.mm
diff --git a/ios/chrome/browser/browser_state/chrome_browser_state.mm b/ios/chrome/browser/browser_state/chrome_browser_state.mm
index d7e709efae63e3cfb608c8c3e78fa208140e4910..a06fca83ef59416e5de7b13e78df9d39885be0ee 100644
--- a/ios/chrome/browser/browser_state/chrome_browser_state.mm
+++ b/ios/chrome/browser/browser_state/chrome_browser_state.mm
@@ -20,6 +20,10 @@
 #include "ios/web/webui/url_data_manager_ios_backend.h"
 #include "net/url_request/url_request_interceptor.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 namespace ios {
 
 ChromeBrowserState::ChromeBrowserState() {}