Roll gn from a0c5124a5060 to 20a6b6d6b824

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Cq-Include-Trybots: luci.dart.try:dart-sdk-linux-try;luci.dart.try:dart-sdk-linux-arm64-try;luci.dart.try:dart-sdk-mac-try;luci.dart.try:dart-sdk-mac-arm64-try;luci.dart.try:dart-sdk-win-try
Change-Id: I249d75f9555815b9ab37bc3ddd7f232dc61200cf
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/460320
Reviewed-by: Alexander Aprelev <[email protected]>
Reviewed-by: Brian Quinlan <[email protected]>
Commit-Queue: Alexander Aprelev <[email protected]>

Author: DEPS Autoroller

DEPS

@@ -78,7 +78,7 @@ vars = {
   "clang_version": "git_revision:f77ce52b56d025399f489a8c0aad8c18c4b06045",
 
   # https://chrome-infra-packages.appspot.com/p/gn/gn
-  "gn_version": "git_revision:a0c5124a50608595a9aadebc4297e854ebd32c53",
+  "gn_version": "git_revision:20a6b6d6b824e78aa4991e8f06b5a8d52576b993",
 
   "reclient_version": "re_client_version:28341fc74c68f05a5c8be35160ada940c4edb969",
   "download_reclient": True,