Revamp the MigrateBrowserPrefs and MigrateUserPrefs code.

chrome/browser/ui/prefs/prefs_tab_helper.cc

Index: chrome/browser/ui/prefs/prefs_tab_helper.cc
diff --git a/chrome/browser/ui/prefs/prefs_tab_helper.cc b/chrome/browser/ui/prefs/prefs_tab_helper.cc
index b1c23df4a8bd482a36eb1ed74f70b1e4c567ab19..a4100ab0547134ae2a9889f053b181d2b0f47591 100644
--- a/chrome/browser/ui/prefs/prefs_tab_helper.cc
+++ b/chrome/browser/ui/prefs/prefs_tab_helper.cc
@@ -507,7 +507,6 @@ void PrefsTabHelper::InitIncognitoUserPrefStore(
   // List of keys that cannot be changed in the user prefs file by the incognito
   // profile.  All preferences that store information about the browsing history
   // or behavior of the user should have this property.
-  pref_store->RegisterOverlayPref(prefs::kBrowserWindowPlacement);
   pref_store->RegisterOverlayPref(prefs::kSaveFileDefaultDirectory);
 #if defined(OS_ANDROID) || defined(OS_IOS)
   pref_store->RegisterOverlayPref(prefs::kProxy);