[ObjC ARC] Converts ios/chrome/app/application_delegate:application_delegate_internal to ARC.

ios/chrome/app/application_delegate/background_activity.mm

Index: ios/chrome/app/application_delegate/background_activity.mm
diff --git a/ios/chrome/app/application_delegate/background_activity.mm b/ios/chrome/app/application_delegate/background_activity.mm
index f5c552505e36658074923fc4bc3158a84f7ba552..2949ee515adfc55d49d80d2cbca2284c4d85452b 100644
--- a/ios/chrome/app/application_delegate/background_activity.mm
+++ b/ios/chrome/app/application_delegate/background_activity.mm
@@ -4,7 +4,6 @@
 
 #import "ios/chrome/app/application_delegate/background_activity.h"
 
-#import "base/ios/weak_nsobject.h"
 #include "base/metrics/histogram_macros.h"
 #include "base/metrics/user_metrics.h"
 #include "base/metrics/user_metrics_action.h"
@@ -16,6 +15,10 @@
 #import "ios/chrome/browser/metrics/previous_session_info.h"
 #include "url/gurl.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 namespace {
 // Key of the UMA Startup.BackgroundFetchTimeInterval histogram.
 const char* const kUMAMobileBackgroundFetchTimeInterval =