Allow generation of a chromium outdirs when enable_extensions=false

extensions/common/BUILD.gn

 # Copyright 2014 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")
 import("//mojo/public/tools/bindings/mojom.gni")
 
+# TODO(crbug.com/731689): Assert that extensions are enabled.
+
 source_set("common_constants") {
   sources = [
     "constants.cc",
     "constants.h",
   ]
 
   # TODO(jschuh): crbug.com/167187 fix size_t to int truncations.
   configs += [ "//build/config/compiler:no_size_t_to_int_warning" ]
 }
 
@@ skipping 368 matching lines @@
       "//ui/base",
       "//ui/gfx",
       "//url",
     ]
 
     if (is_chromeos) {
       sources += [ "manifest_handlers/action_handlers_handler_unittest.cc" ]
     }
   }
 }  # enable_extensions