Separates PreviousSessionInfo testing interface

ios/chrome/browser/metrics/previous_session_info.mm

 // Copyright 2015 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/browser/metrics/previous_session_info.h"
 
 #include "base/logging.h"
 #include "base/mac/scoped_nsobject.h"
 #include "base/strings/sys_string_conversions.h"
 #include "components/version_info/version_info.h"
+#import "ios/chrome/browser/metrics/previous_session_info_private.h"
 
 namespace {
 
 // Key in the NSUserDefaults for a string value that stores the version of the
 // last session.
 NSString* const kLastRanVersion = @"LastRanVersion";
 
 }  // namespace
 
 namespace previous_session_info_constants {
@@ skipping 84 matching lines @@
 
   NSUserDefaults* defaults = [NSUserDefaults standardUserDefaults];
   [defaults
       removeObjectForKey:previous_session_info_constants::
                              kDidSeeMemoryWarningShortlyBeforeTerminating];
   // Save critical state information for crash detection.
   [defaults synchronize];
 }
 
 @end