[mojo] do forward declaration of non-variant enum in variant header

non variant enum is used for alias in variant header
https://cs.chromium.org/chromium/src/out/Debug/gen/third_party/blink/public/mojom/cache_storage/cache_storage.mojom-blink-forward.h?g=0&l=48

This is found by wanderview@chromium.org in
https://bugs.chromium.org/p/chromium/issues/detail?id=922875#c104

Bug: 922875
Change-Id: I94e0c0063d067d085bc964ee37996a9eb40ec2c8
Reviewed-on: https://chromium-review.googlesource.com/c/1481146
Auto-Submit: Takuto Ikuta <tikuta@chromium.org>
Commit-Queue: Ken Rockot <rockot@google.com>
Reviewed-by: Ken Rockot <rockot@google.com>
Cr-Commit-Position: refs/heads/master@{#634699}

mojo/public/tools/bindings/generators/cpp_templates/module-forward.h.tmpl

@@ -72,6 +72,25 @@ namespace {{variant}} {
 {%-   endfor %}
 {%- endif %}
 
+{%- if variant %}
+{%-   for namespace in namespaces_as_array %}
+namespace {{namespace}} {
+{%-   endfor %}
+
+{%- from "enum_macros.tmpl" import enum_forward%}
+{%- for enum in all_enums %}
+{%-   if enum|is_native_only_kind %}
+using {{enum|get_name_for_kind(flatten_nested_kind=True)}} = mojo::NativeEnum;
+{%-   else %}
+{{enum_forward(enum)}}
+{%-   endif %}
+{%- endfor %}
+
+{%-   for namespace in namespaces_as_array %}
+}  // namespace {{namespace}}
+{%-   endfor %}
+{%- endif %}
+
 {{namespace_begin()}}
 
 {#- These are non-variant header only. #}