Allow generation of a chromium outdirs when enable_extensions=false

extensions/browser/api/display_source/BUILD.gn

 # Copyright 2016 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/config/features.gni")
 import("//extensions/features/features.gni")
 
+assert(enable_extensions,
+       "Cannot depend on extensions because enable_extensions=false.")
+
 source_set("display_source") {
   sources = [
     "display_source_api.cc",
     "display_source_api.h",
     "display_source_connection_delegate.cc",
     "display_source_connection_delegate.h",
     "display_source_connection_delegate_factory.cc",
     "display_source_connection_delegate_factory.h",
     "display_source_event_router.cc",
     "display_source_event_router.h",
@@ skipping 13 matching lines @@
   }
 
   deps = [
     "//extensions/common/api",
   ]
 
   public_deps = [
     "//extensions/browser:browser_sources",
   ]
 }