Rewire the local sync switches through the pref store.

components/sync/base/pref_names.cc

Index: components/sync/base/pref_names.cc
diff --git a/components/sync/base/pref_names.cc b/components/sync/base/pref_names.cc
index c98314b3a03d7bad54c2bef0a0042cc096ddd87f..5008badad6be8ef045d810e2ff36b618fac77ee3 100644
--- a/components/sync/base/pref_names.cc
+++ b/components/sync/base/pref_names.cc
@@ -124,6 +124,14 @@ const char kSyncPassphraseEncryptionTransitionInProgress[] =
 const char kSyncNigoriStateForPassphraseTransition[] =
     "sync.nigori_state_for_passphrase_transition";
 
+// Enabled the local sync backend implemented by the LoopbackServer.
+const char kEnableLocalSyncBackend[] = "sync.enable_local_sync_backend";
+
+// Specifies the local sync backend directory. The name is chosen to mimic
+// user-data-dir etc. This flag only matters if the enable-local-sync-backend
+// flag is present.
+const char kLocalSyncBackendDir[] = "sync.local_sync_backend_dir";
+
 }  // namespace prefs
 
 }  // namespace syncer