[ObjC ARC] Converts ios/chrome/browser/crash_report to ARC

ios/chrome/browser/crash_report/crash_report_background_uploader.mm

Index: ios/chrome/browser/crash_report/crash_report_background_uploader.mm
diff --git a/ios/chrome/browser/crash_report/crash_report_background_uploader.mm b/ios/chrome/browser/crash_report/crash_report_background_uploader.mm
index 450861bbd4ce990de38b4ff51ae601bf347b6499..dc1fa99972b2b1111941884a4bb4ee5882f5584c 100644
--- a/ios/chrome/browser/crash_report/crash_report_background_uploader.mm
+++ b/ios/chrome/browser/crash_report/crash_report_background_uploader.mm
@@ -16,6 +16,10 @@
 #import "breakpad/src/client/ios/BreakpadController.h"
 #include "ios/chrome/browser/experimental_flags.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 using base::UserMetricsAction;
 
 namespace {
@@ -65,8 +69,7 @@ NSString* CreateSessionIdentifierFromTask(NSURLSessionTask* task) {
 
 - (void)setSessionCompletionHandler:(ProceduralBlock)completionHandler {
   DCHECK(completionHandler);
-  _sessionCompletionHandler.reset(completionHandler,
-                                  base::scoped_policy::RETAIN);
+  _sessionCompletionHandler.reset(completionHandler);
   _didFinishEventsCalled = NO;
 }