Revamp the MigrateBrowserPrefs and MigrateUserPrefs code.

chrome/common/pref_names.h

Index: chrome/common/pref_names.h
diff --git a/chrome/common/pref_names.h b/chrome/common/pref_names.h
index 319d971f8feb12d6dc7a1ddfb280d5a5baa9149c..23fc5ac566a047df9e22c07e96116e5bb76ad3f3 100644
--- a/chrome/common/pref_names.h
+++ b/chrome/common/pref_names.h
@@ -19,6 +19,8 @@
 #include "components/password_manager/core/common/password_manager_pref_names.h"
 #include "components/signin/core/common/signin_pref_names.h"
 
+// TODO(rkaplow): Move obsolete prefs only needed in brwoser_prefs.cc to
+// browser_prefs.cc.
 namespace prefs {
 
 // Profile prefs. Please add Local State prefs below instead.
@@ -152,9 +154,7 @@ extern const char kConfirmToQuitEnabled[];
 extern const char kCookieBehavior[];  // OBSOLETE
 extern const char kPromptForDownload[];
 extern const char kAlternateErrorPagesEnabled[];
-extern const char kDnsStartupPrefetchList[];  // OBSOLETE
 extern const char kDnsPrefetchingStartupList[];
-extern const char kDnsHostReferralList[];  // OBSOLETE
 extern const char kDnsPrefetchingHostReferralList[];
 extern const char kDisableSpdy[];
 extern const char kHttpServerProperties[];
@@ -165,7 +165,6 @@ extern const char kDisabledSchemes[];
 extern const char kLastPolicyCheckTime[];
 #endif
 extern const char kInstantUIZeroSuggestUrlPrefix[];
-extern const char kMultipleProfilePrefMigration[];
 extern const char kNetworkPredictionEnabled[];
 extern const char kNetworkPredictionOptions[];
 extern const char kDefaultAppsInstallState[];