Port aom_reader_tell() support

This commit ports the following from aom/master:
4c46278 Add aom_reader_tell() support.
b9c9935 Remove an erroneous declaration.
56c9c3b Fix ANS build.

Change-Id: I59bd910f58c218c649a1de2a7b5fae0397e13cb1
diff --git a/aom_dsp/dkboolreader.c b/aom_dsp/dkboolreader.c
index 8ec7ffc..c3b7782 100644
--- a/aom_dsp/dkboolreader.c
+++ b/aom_dsp/dkboolreader.c
@@ -29,7 +29,7 @@
     return 1;
   } else {
     r->buffer_end = buffer + size;
-    r->buffer = buffer;
+    r->buffer_start = r->buffer = buffer;
     r->value = 0;
     r->count = -8;
     r->range = 255;