GN: Throw an error overwriting a nonempty scope.

mojo/public/tools/bindings/chromium_bindings_configuration.gni

 # 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.
 
 _typemap_imports = [
   "//cc/ipc/typemaps.gni",
   "//chrome/browser/media/router/mojo/typemaps.gni",
   "//components/arc/common/typemaps.gni",
   "//components/typemaps.gni",
   "//content/common/bluetooth/typemaps.gni",
   "//device/bluetooth/public/interfaces/typemaps.gni",
   "//gpu/ipc/common/typemaps.gni",
   "//media/mojo/interfaces/typemaps.gni",
   "//mojo/common/typemaps.gni",
   "//mojo/public/cpp/bindings/tests/chromium_typemaps.gni",
   "//services/ui/public/interfaces/display/typemaps.gni",
   "//skia/public/interfaces/typemaps.gni",
   "//ui/events/devices/mojo/typemaps.gni",
   "//ui/events/mojo/typemaps.gni",
   "//ui/gfx/typemaps.gni",
   "//url/mojo/typemaps.gni",
 ]
 _typemaps = []
 
 foreach(typemap_import, _typemap_imports) {
+  # Avoid reassignment error by assigning to empty scope first.
+  _imported = {
+  }
   _imported = read_file(typemap_import, "scope")
   _typemaps += _imported.typemaps
 }
 
 typemaps = []
 foreach(typemap, _typemaps) {
   typemaps += [ read_file(typemap, "scope") ]
 }