ext/libgav1.cmd: sync to 4a89dc3, update abseil-cpp

update abseil-cpp -b lts_2020_09_23
update libgav1 to 4a89dc3

now in sync with docker/build.sh

ext/libgav1.cmd

diff --git a/ext/libgav1.cmd b/ext/libgav1.cmd
index cd163cd..e54726f 100755
--- a/ext/libgav1.cmd
+++ b/ext/libgav1.cmd
@@ -11,8 +11,8 @@
 git clone --single-branch https://chromium.googlesource.com/codecs/libgav1
 
 cd libgav1
-git checkout 45a1d76
-git clone -b 20200225.2 --depth 1 https://github.com/abseil/abseil-cpp.git third_party/abseil-cpp
+git checkout 4a89dc3
+git clone -b lts_2020_09_23 --depth 1 https://github.com/abseil/abseil-cpp.git third_party/abseil-cpp
 mkdir build
 cd build