Add bitrate accuracy flag.

Intended to be used with the Improve Bitrate Accuracy project.

Change-Id: I91121f8ae910e582498d04bc696a98013d3c5622
BUG=aomedia:3045

build/cmake/aom_config_defaults.cmake

diff --git a/build/cmake/aom_config_defaults.cmake b/build/cmake/aom_config_defaults.cmake
index 8f96d13..8bf12c5 100644
--- a/build/cmake/aom_config_defaults.cmake
+++ b/build/cmake/aom_config_defaults.cmake
@@ -146,6 +146,8 @@
                    "Build with ML-based partitioning for Real Time.")
 set_aom_config_var(CONFIG_PARTITION_SEARCH_ORDER 0
                    "AV1 experiment with alternative partition search order.")
+set_aom_config_var(CONFIG_BITRATE_ACCURACY 0
+                   "AV1 experiment to improve bitrate accuracy.")
 
 #
 # Variables in this section control optional features of the build system.