[ObjC ARC] Converts ios/chrome/test/app:test_support to ARC.

ios/chrome/test/app/histogram_test_util.mm

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #import "ios/chrome/test/app/histogram_test_util.h"
 
 #import <Foundation/Foundation.h>
 
 #include "base/memory/ptr_util.h"
 #include "base/metrics/histogram_macros.h"
 #include "base/metrics/histogram_samples.h"
 #include "base/metrics/metrics_hashes.h"
 #include "base/metrics/sample_map.h"
 #include "base/metrics/statistics_recorder.h"
 #include "base/stl_util.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 namespace {
 base::HistogramBase* FindHistogram(const std::string& name,
                                    FailureBlock failure_block) {
   base::HistogramBase* histogram =
       base::StatisticsRecorder::FindHistogram(name);
   if (!histogram && failure_block) {
     failure_block([NSString
         stringWithFormat:@"Histogram %s does not exist", name.c_str()]);
   }
   return histogram;
@@ skipping 151 matching lines @@
     failure_block(
         [NSString stringWithFormat:@"Histogram \"%s\" doe not have the "
                                     "right total number of samples(%d). It has "
                                     "(%d).",
                                    name.c_str(), expected_count, actual_count]);
   }
   return NO;
 }
 
 }  // namespace chrome_test_util