Roll gn from ed1abc107815 to ab638bd7cbb9

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
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Cq-Include-Trybots: luci.dart.try:dart-sdk-linux-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: I50244b986b8cbaaad53a92366ef29f24e66f7440
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/405982
Commit-Queue: Brian Quinlan <[email protected]>
Reviewed-by: Brian Quinlan <[email protected]>
Reviewed-by: Alexander Aprelev <[email protected]>

Author: DEPS Autoroller

DEPS

@@ -79,7 +79,7 @@ vars = {
   "clang_version": "git_revision:84af3ee5124de3385b829c3a9980fd734f0d92e8",
 
   # https://chrome-infra-packages.appspot.com/p/gn/gn
-  "gn_version": "git_revision:ed1abc107815210dc66ec439542bee2f6cbabc00",
+  "gn_version": "git_revision:ab638bd7cbb9ac8468bf2fbe60c74ed4706a14a7",
 
   "reclient_version": "git_revision:c7349324c93c6e0d85bc1e00b5d7526771006ea0",
   "download_reclient": True,