Fix ext-tile and obu

Fix the broken unit test when ext-tile and obu are both turned on.
The test failure was due to the merge of CONFIG_ADD_4BYTES_OBUSIZE.
This solves the unit test failure cases when cm->large_scale_tile
is on.

BUG=aomedia:1179

Change-Id: I7b4a5720e493db2a7d914fc9f8b886a67b1e85cd

av1/encoder/bitstream.c

diff --git a/av1/encoder/bitstream.c b/av1/encoder/bitstream.c
index 7e7c550..b27b906 100644
--- a/av1/encoder/bitstream.c
+++ b/av1/encoder/bitstream.c
@@ -4540,6 +4540,7 @@
 
     uint32_t tg_hdr_size =
         write_obu_header(OBU_TILE_GROUP, 0, data + PRE_OBU_SIZE_BYTES);
+    tg_hdr_size += PRE_OBU_SIZE_BYTES;
     data += tg_hdr_size;
 
     int tile_size_bytes;