Force HBD flag when FORCE_HIGHBITDEPTH_DECODING is set.

BUG=b/139764277,aomedia:2500,aomedia:2501,aomedia:2502

Change-Id: I88280c671d0681c9ba4575f5271ac3b2cc80d1ed
diff --git a/build/cmake/aom_configure.cmake b/build/cmake/aom_configure.cmake
index 30d95e7..635b57a 100644
--- a/build/cmake/aom_configure.cmake
+++ b/build/cmake/aom_configure.cmake
@@ -30,6 +30,11 @@
   set(FORCE_HIGHBITDEPTH_DECODING 1 CACHE NUMBER "${cmake_cmdline_helpstring}")
 endif()
 
+if(FORCE_HIGHBITDEPTH_DECODING AND NOT CONFIG_AV1_HIGHBITDEPTH)
+  change_config_and_warn(CONFIG_AV1_HIGHBITDEPTH 1
+                         "FORCE_HIGHBITDEPTH_DECODING")
+endif()
+
 # Generate the user config settings.
 list(APPEND aom_build_vars ${AOM_CONFIG_VARS} ${AOM_OPTION_VARS})
 foreach(cache_var ${aom_build_vars})