Reland of Have gn gen assert that the output directory does not have GYP artifacts (patchset #1 id:1 of https://codereview.chromium.org/1602393002/ )

Reason for revert:
Verified manually (via ssh) that patch works fine on a trybot that broke last time. Perhaps more time just needed to pass?

Original issue's description:
> Revert of Have gn gen assert that the output directory does not have GYP artifacts (patchset #2 id:20001 of https://codereview.chromium.org/1601893002/ )
>
> Reason for revert:
> arm-builder-rel, arm64-builder-rel, gn-release-builder-l consistently fail the assertion added in this change.
>
> Original issue's description:
> > Have gn gen assert that the output directory does not have GYP artifacts
> >
> > This can often lead to compile errors.
> >
> > BUG=578862
> >
> > Committed: https://crrev.com/5915049689627393a3eb8f531f3a361eedcfbce0
> > Cr-Commit-Position: refs/heads/master@{#370210}
>
> TBR=brettw@chromium.org,agrieve@chromium.org
> # Skipping CQ checks because original CL landed less than 1 days ago.
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=578862
>
> Committed: https://crrev.com/caf664a996be25ec5f567f183dd897de4abff952
> Cr-Commit-Position: refs/heads/master@{#370288}

TBR=brettw@chromium.org,jbudorick@chromium.org,mdjones@chromium.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=578862

Review URL: https://codereview.chromium.org/1702013002

Cr-Commit-Position: refs/heads/master@{#375598}

1 files changed, 1 insertions, 0 deletions

diff --git a/.gn b/.gn
index 033b3ad..e32288d 100644
--- a/.gn
+++ b/.gn
@@ -131,6 +131,7 @@ check_targets = [
 exec_script_whitelist = [
   "//android_webview/BUILD.gn",
   "//ash/BUILD.gn",
+  "//build/config/BUILD.gn",
   "//build/config/android/config.gni",
   "//build/config/android/internal_rules.gni",
   "//build/config/android/rules.gni",