mojom: Add associative arrays to the mojom language.

mojo/public/cpp/bindings/tests/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.
 
 # GYP version: mojo/mojo_base.gyp:mojo_public_bindings_unittests
 test("mojo_public_bindings_unittests") {
   sources = [
     "array_unittest.cc",
     "bounds_checker_unittest.cc",
     "buffer_unittest.cc",
     "connector_unittest.cc",
     "handle_passing_unittest.cc",
     "interface_ptr_unittest.cc",
+    "map_unittest.cc",
     "request_response_unittest.cc",
     "router_unittest.cc",
     "sample_service_unittest.cc",
     "serialization_warning_unittest.cc",
     "string_unittest.cc",
     "struct_unittest.cc",
     "type_conversion_unittest.cc",
     "validation_unittest.cc",
   ]
 
@@ skipping 13 matching lines @@
 source_set("mojo_public_bindings_test_utils") {
   sources = [
     "validation_test_input_parser.cc",
     "validation_test_input_parser.h",
   ]
 
   deps = [
     "//mojo/public/c/system",
   ]
 }