Revamp the MigrateBrowserPrefs and MigrateUserPrefs code.

chrome/browser/profiles/profile_impl.cc

 // Copyright (c) 2012 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 "chrome/browser/profiles/profile_impl.h"
 
 #include <vector>
 
 #include "base/bind.h"
 #include "base/callback.h"
@@ skipping 837 matching lines @@
 }
 
 void ProfileImpl::OnPrefsLoaded(bool success) {
   TRACE_EVENT0("browser", "ProfileImpl::OnPrefsLoaded")
   if (!success) {
     if (delegate_)
       delegate_->OnProfileCreated(this, false, false);
     return;
   }
 
-  // TODO(mirandac): remove migration code after 6 months (crbug.com/69995).
-  if (g_browser_process->local_state())
-    chrome::MigrateBrowserPrefs(this, g_browser_process->local_state());
   // TODO(ivankr): remove cleanup code eventually (crbug.com/165672).

[gab, 2015/02/20 19:04:03]

No, we shouldn't get rid of this hook.

If anything we should leave both of these hooks here (removing the TODOs).

And instead cleanup the old pref clearance (git blame to see how old) in those
methods themselves (even if that means leaving them empty).

(also changing the API to make it clear that those methods should be used to
clear old prefs and that their content should be cycled over time)

[rkaplow, 2015/02/20 19:08:53]

Hm, ok. 

How long should pref migration live for? I.e. what should the cutoff be?

   chrome::MigrateUserPrefs(this);
 
   // |kSessionExitType| was added after |kSessionExitedCleanly|. If the pref
   // value is empty fallback to checking for |kSessionExitedCleanly|.
   const std::string exit_type_pref_value(
       prefs_->GetString(prefs::kSessionExitType));
   if (exit_type_pref_value.empty()) {
     last_session_exit_type_ =
         prefs_->GetBoolean(prefs::kSessionExitedCleanly) ?
           EXIT_NORMAL : EXIT_CRASHED;
@@ skipping 451 matching lines @@
 ProfileImpl::CreateDomainReliabilityMonitor(PrefService* local_state) {
   domain_reliability::DomainReliabilityService* service =
       domain_reliability::DomainReliabilityServiceFactory::GetInstance()->
           GetForBrowserContext(this);
   if (!service)
     return scoped_ptr<domain_reliability::DomainReliabilityMonitor>();
 
   return service->CreateMonitor(
       BrowserThread::GetMessageLoopProxyForThread(BrowserThread::IO));
 }