Separates PreviousSessionInfo testing interface

ios/chrome/browser/metrics/previous_session_info_unittest.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.
 
 #include "ios/chrome/browser/metrics/previous_session_info.h"
 
 #include "base/strings/sys_string_conversions.h"
 #include "components/version_info/version_info.h"
+#include "ios/chrome/browser/metrics/previous_session_info_private.h"
 #include "testing/gtest/include/gtest/gtest.h"
 #include "testing/gtest_mac.h"
 
 namespace {
 
 // Key in the UserDefaults for a boolean value keeping track of memory warnings.
 NSString* const kDidSeeMemoryWarningShortlyBeforeTerminating =
     previous_session_info_constants::
         kDidSeeMemoryWarningShortlyBeforeTerminating;
 
@@ skipping 161 matching lines @@
       [defaults boolForKey:kDidSeeMemoryWarningShortlyBeforeTerminating]);
 
   // Call the memory warning flag resetter.
   [[PreviousSessionInfo sharedInstance] resetMemoryWarningFlag];
 
   EXPECT_FALSE(
       [defaults boolForKey:kDidSeeMemoryWarningShortlyBeforeTerminating]);
 }
 
 }  // namespace