mojom: Add associative arrays to the mojom language.

mojo/mojo_services.gypi

Index: mojo/mojo_services.gypi
diff --git a/mojo/mojo_services.gypi b/mojo/mojo_services.gypi
index eb9b0db822ad88cfa41fc553652c1f25dd8d1f59..1135aef64f42f4e2215d7917dc459cb262cc9649 100644
--- a/mojo/mojo_services.gypi
+++ b/mojo/mojo_services.gypi
@@ -31,6 +31,10 @@
         'services/public/interfaces/clipboard/clipboard.mojom',
       ],
       'includes': [ 'public/tools/bindings/mojom_bindings_generator.gypi' ],
+      'variables': {
+        # TODO(erg): Expand this as we add map support to other bindings.
+        'mojom_additional_args': ['-g=c++'],
+      },
       'dependencies': [
         'public/mojo_public.gyp:mojo_cpp_bindings',
       ],