[subset] Move DeviceRecord inside of hdmx.

src/hb-ot-hdmx-table.hh

@@ -34,8 +34,13 @@ namespace OT {
 #define HB_OT_TAG_hdmx HB_TAG('h','d','m','x')
 
 
-struct DeviceRecord
+
+struct hdmx
 {
+  static const hb_tag_t tableTag = HB_OT_TAG_glyf;
+
+  struct DeviceRecord
+  {
     struct SubsetView
     {
       const DeviceRecord *source_device_record;
@@ -91,10 +96,7 @@ struct DeviceRecord
     HBUINT8 widths[VAR];
 
     DEFINE_SIZE_MIN (2);
-};
-
-struct hdmx
-{
+  };
 
   inline unsigned int get_size (void) const
   {