Revamp the MigrateBrowserPrefs and MigrateUserPrefs code.

chrome/browser/profiles/profile_impl.cc

Index: chrome/browser/profiles/profile_impl.cc
diff --git a/chrome/browser/profiles/profile_impl.cc b/chrome/browser/profiles/profile_impl.cc
index 4fcb7e476868d7a0b1bcfdf970ca424aa082f8fc..b028168fe8abc27d1832d2676c082144c24a157c 100644
--- a/chrome/browser/profiles/profile_impl.cc
+++ b/chrome/browser/profiles/profile_impl.cc
@@ -855,9 +855,6 @@ void ProfileImpl::OnPrefsLoaded(bool success) {
     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);