Mojo: Simplify typemap usage.

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 = [
+  "//gpu/command_buffer/common/typemaps.gni",
+  "//mojo/public/cpp/bindings/tests/chromium_typemaps.gni",
+  "//url/mojo/typemaps.gni",
+]
+_typemaps = []
+
+foreach(typemap_import, _typemap_imports) {
+  _imported = read_file(typemap_import, "scope")
+  _typemaps += _imported.typemaps
+}
+
+typemaps = []
+foreach(typemap, _typemaps) {
+  typemaps += [ read_file(typemap, "scope") ]
+}