[ObjC ARC] Converts components/history/ios/browser:browser to ARC.

components/history/ios/browser/web_state_top_sites_observer.mm

Index: components/history/ios/browser/web_state_top_sites_observer.mm
diff --git a/components/history/ios/browser/web_state_top_sites_observer.mm b/components/history/ios/browser/web_state_top_sites_observer.mm
index ac95a554411fbc348a43c32162bfb7f1c9a1dc27..cdddbd9a240dca27a530a50fef4a762e923dd77f 100644
--- a/components/history/ios/browser/web_state_top_sites_observer.mm
+++ b/components/history/ios/browser/web_state_top_sites_observer.mm
@@ -9,6 +9,10 @@
 #include "ios/web/public/load_committed_details.h"
 #include "ios/web/public/navigation_item.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 DEFINE_WEB_STATE_USER_DATA_KEY(history::WebStateTopSitesObserver);
 
 namespace history {