Fix 'gn gen' when gn is built in debug config

Fixes this DCHECK:
https://cs.chromium.org/chromium/gn/tools/gn/source_file.cc?q=source_file.cc&sq=package:chromium&dr&l=21

BUG=None

Change-Id: Ide60a650c800d4a0981b4f28cf0427fa91616464
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1613888
Reviewed-by: Brian Sheedy <bsheedy@chromium.org>
Commit-Queue: Brian Sheedy <bsheedy@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Auto-Submit: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#660103}
diff --git a/tools/perf/contrib/vr_benchmarks/BUILD.gn b/tools/perf/contrib/vr_benchmarks/BUILD.gn
index 758057a..51644e0 100644
--- a/tools/perf/contrib/vr_benchmarks/BUILD.gn
+++ b/tools/perf/contrib/vr_benchmarks/BUILD.gn
@@ -72,7 +72,7 @@
     "//chrome/browser/resources/vr/assets/vr_assets_component_files.json",
   ]
   outputs = [
-    "$target_gen_dir/vr_assets_profile/",
+    "$target_gen_dir/vr_assets_profile",
   ]
   args = [
     "--output",