Add an experimental flag FRAME_REFS_SIGNALING

This flag enables an extra bit in the frame header to indicate the
signaling choice of the reference frames in the frame header.

Change-Id: I67ef0cbb0673bf877189236b5060f2a854a05d11

build/cmake/aom_config_defaults.cmake

diff --git a/build/cmake/aom_config_defaults.cmake b/build/cmake/aom_config_defaults.cmake
index 7852c1c..3cacf06 100644
--- a/build/cmake/aom_config_defaults.cmake
+++ b/build/cmake/aom_config_defaults.cmake
@@ -116,6 +116,7 @@
 set(CONFIG_FILM_GRAIN 0 CACHE NUMBER "AV1 experiment flag.")
 set(CONFIG_FILTER_INTRA 1 CACHE NUMBER "AV1 experiment flag.")
 set(CONFIG_FP_MB_STATS 0 CACHE NUMBER "AV1 experiment flag.")
+set(CONFIG_FRAME_REFS_SIGNALING 0 CACHE NUMBER "AV1 experiment flag.")
 set(CONFIG_FRAME_SIZE 1 CACHE NUMBER "AV1 experiment flag.")
 set(CONFIG_FWD_KF 0 CACHE NUMBER "AV1 experiment flag.")
 set(CONFIG_HASH_ME 0 CACHE NUMBER "AV1 experiment flag.")