GN: Throw an error overwriting a nonempty scope.

mojo/public/tools/bindings/blink_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.
 
 variant = "blink"
 
 for_blink = true
 
 _typemap_imports = [
   "//mojo/public/cpp/bindings/tests/blink_typemaps.gni",
   "//third_party/WebKit/Source/platform/mojo/blink_typemaps.gni",
   "//third_party/WebKit/public/blink_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") ]
 }
 
 blacklist = [
   # TODO(sammc): Remove the following once |for_blink| bindings support WTF
   # maps with enum keys. See https://crbug.com/583738.
   "//mojo/public/interfaces/bindings/tests/validation_test_interfaces.mojom",
 ]