Allow encoder to count the number of frame headers

The number of frame headers will be used to calculate frame header rate,
which is part of the bitstream level definition.

BUG=aomedia:2332

Change-Id: I69c8b21654fbba84476d38b233052367dc1e3bf8
diff --git a/av1/encoder/encoder.c b/av1/encoder/encoder.c
index d138cd6..542fcd5 100644
--- a/av1/encoder/encoder.c
+++ b/av1/encoder/encoder.c
@@ -5694,7 +5694,7 @@
   if (payload_offset + sequence_header_size > sizeof(header_buf)) return NULL;
   memmove(&header_buf[payload_offset], &header_buf[0], sequence_header_size);
 
-  if (write_obu_header(OBU_SEQUENCE_HEADER, 0, &header_buf[0]) !=
+  if (av1_write_obu_header(cpi, OBU_SEQUENCE_HEADER, 0, &header_buf[0]) !=
       obu_header_size) {
     return NULL;
   }