read_obu_header set temporal/spatial_layer_id to 0

Have read_obu_header() set the temporal_layer_id and spatial_layer_id
fields of ObuHeader to 0 when has_extension is false. This implements
the following descriptions in Section 6.2.3 of the AV1 spec:

  ... When temporal_id is not present, temporal_id is inferred to be
  equal to 0.

  ... When spatial_id is not present, spatial_id is inferred to be
  equal to 0.

BUG=aomedia:2801

Change-Id: I537d8f53e1d7a853eefbf28ab6081f97833146c6

av1/common/obu_util.c

diff --git a/av1/common/obu_util.c b/av1/common/obu_util.c
index 7d2694b..3e35d71 100644
--- a/av1/common/obu_util.c
+++ b/av1/common/obu_util.c
@@ -89,6 +89,9 @@
       // extension_header_reserved_3bits must be set to 0.
       return AOM_CODEC_CORRUPT_FRAME;
     }
+  } else {
+    header->temporal_layer_id = 0;
+    header->spatial_layer_id = 0;
   }
 
   return AOM_CODEC_OK;

av1/common/obu_util.h

diff --git a/av1/common/obu_util.h b/av1/common/obu_util.h
index 7c56904..adf3568 100644
--- a/av1/common/obu_util.h
+++ b/av1/common/obu_util.h
@@ -22,9 +22,9 @@
                 // optional OBU extension header) in the bitstream.
   OBU_TYPE type;
   int has_size_field;
-  int has_extension;
-  // The following fields come from the OBU extension header and therefore are
-  // only used if has_extension is true.
+  int has_extension;  // Whether the optional OBU extension header is present.
+  // The following fields come from the OBU extension header. They are set to 0
+  // if has_extension is false.
   int temporal_layer_id;
   int spatial_layer_id;
 } ObuHeader;