Roll gn from 78e67afc82fa to 068dd892acb2

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 dart-engprod@google.com,dart-vm-gardener@rotations.google.com,dart-vm-team@google.com 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: I3666c8515b37c13a8ebe6b98413e8d68b9a18fd9
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/502640
Commit-Queue: Alexander Markov <alexmarkov@google.com>
Reviewed-by: Ryan Macnak <rmacnak@google.com>
Reviewed-by: Liam Appelbe <liama@google.com>

DEPS

@@ -83,7 +83,7 @@ vars = {
   "clang_version": "git_revision:a3f244e2d5550e110daee562b88bee247b75d66c",
 
   # https://chrome-infra-packages.appspot.com/p/gn/gn
-  "gn_version": "git_revision:78e67afc82faf52b2f7025265d412fd7418d88c2",
+  "gn_version": "git_revision:068dd892acb22dc21d9e17691266c8499ba2d6c8",
 
   "reclient_version": "re_client_version:28341fc74c68f05a5c8be35160ada940c4edb969",
   "download_reclient": True,