[Sync] Add support for proxy types

chrome/browser/sync/backend_migrator.cc

Index: chrome/browser/sync/backend_migrator.cc
diff --git a/chrome/browser/sync/backend_migrator.cc b/chrome/browser/sync/backend_migrator.cc
index e6c50020f33aa9cecddf72777bd0882c3e5f2533..8d5d1ea710439b655c61ddb45ef5fdc0108bbb64 100644
--- a/chrome/browser/sync/backend_migrator.cc
+++ b/chrome/browser/sync/backend_migrator.cc
@@ -46,6 +46,10 @@ BackendMigrator::~BackendMigrator() {
 #define SDVLOG(verbose_level) DVLOG(verbose_level) << name_ << ": "
 
 void BackendMigrator::MigrateTypes(syncer::ModelTypeSet types) {
+  if (!syncer::Intersection(types, syncer::VirtualTypes()).Empty()) {
+    LOG(WARNING) << "Virtual type migration attempted, dropping local types.";
+    types.RemoveAll(syncer::VirtualTypes());
+  }
   const ModelTypeSet old_to_migrate = to_migrate_;
   to_migrate_.PutAll(types);
   SDVLOG(1) << "MigrateTypes called with " << ModelTypeSetToString(types)