Add support for (de)serializing HeadsUpDisplayLayer

cc/proto/layer.proto

Index: cc/proto/layer.proto
diff --git a/cc/proto/layer.proto b/cc/proto/layer.proto
index 41b3f2d8086e319e7672214fa9500e90bb84cefd..bc1255daa52ee6b045e9f002624c6bf450d09d99 100644
--- a/cc/proto/layer.proto
+++ b/cc/proto/layer.proto
@@ -26,6 +26,7 @@ enum LayerType {
   UNKNOWN = 0;
   LAYER = 1;
   PICTURE_LAYER = 2;
+  HEADS_UP_DISPLAY_LAYER = 3;
 
   // TODO(nyquist): Add the rest of the necessary LayerTypes.
 };
@@ -138,4 +139,4 @@ message PictureLayerProperties {
   optional bool nearest_neighbor = 5;
 
   optional int64 update_source_frame_number = 6;
-}
+}