Roll gn from a156d0b1306b to 6e0b557db44b

If this roll has caused a breakage, revert this CL and stop the roller
using the controls here:
https://autoroll.skia.org/r/gn-dart-sdk
Please CC [email protected],[email protected],[email protected] on the revert to ensure that a human
is aware of the problem.

To file a bug in gn: https://bugs.chromium.org/p/gn/issues/list
To file a bug in Dart SDK: https://github.com/dart-lang/sdk/issues

To report a problem with the AutoRoller itself, please file a bug:
https://issues.skia.org/issues/new?component=1389291&template=1850622

Documentation for the AutoRoller is here:
https://skia.googlesource.com/buildbot/+doc/main/autoroll/README.md

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: I228446374a9866eae927e4c62815ec3d3fe6c486
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/465280
Reviewed-by: Brian Quinlan <[email protected]>
Commit-Queue: Brian Quinlan <[email protected]>
Reviewed-by: Ryan Macnak <[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:a156d0b1306b09bf676a72e345ca978ef6093d95",
+  "gn_version": "git_revision:6e0b557db44b3c164094e57687d20ba036a80667",
 
   "reclient_version": "re_client_version:28341fc74c68f05a5c8be35160ada940c4edb969",
   "download_reclient": True,