Allow generation of a chromium outdirs when enable_extensions=false

extensions/browser/api/metrics_private/BUILD.gn

Index: extensions/browser/api/metrics_private/BUILD.gn
diff --git a/extensions/browser/api/metrics_private/BUILD.gn b/extensions/browser/api/metrics_private/BUILD.gn
index 75656bd3b8bde5c4036e67c3f500b4b8c59f9342..39a60631d9595a86dcbe9e8933a925ca77bb8963 100644
--- a/extensions/browser/api/metrics_private/BUILD.gn
+++ b/extensions/browser/api/metrics_private/BUILD.gn
@@ -2,6 +2,11 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import("//extensions/features/features.gni")
+
+assert(enable_extensions,
+       "Cannot depend on extensions because enable_extensions=false.")
+
 source_set("metrics_private") {
   sources = [
     "metrics_private_api.cc",