sync: Add DeviceInfo protobuf and supporting code

sync/internal_api/base_node.cc

Index: sync/internal_api/base_node.cc
diff --git a/sync/internal_api/base_node.cc b/sync/internal_api/base_node.cc
index 9595d293456a7f87b9fc93c4aef6edcffa667d43..f1ada207bef0ed607452900d0ade29e28de7283f 100644
--- a/sync/internal_api/base_node.cc
+++ b/sync/internal_api/base_node.cc
@@ -332,6 +332,11 @@ const sync_pb::SessionSpecifics& BaseNode::GetSessionSpecifics() const {
   return GetEntitySpecifics().session();
 }
 
+const sync_pb::DeviceInfoSpecifics& BaseNode::GetDeviceInfoSpecifics() const {
+  DCHECK_EQ(GetModelType(), DEVICE_INFO);
+  return GetEntitySpecifics().device_info();
+}
+
 const sync_pb::EntitySpecifics& BaseNode::GetEntitySpecifics() const {
   return GetUnencryptedSpecifics(GetEntry());
 }