[mojo] forward declare non-variant enum before WTF enum hashing

Fix for
https://bugs.chromium.org/p/chromium/issues/detail?id=922875#c107

Bug: 922875
Change-Id: Ifddc1f7ebf14117ed734bfbab6fd16b13632979a
Reviewed-on: https://chromium-review.googlesource.com/c/1488395
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@{#635429}

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

@@ -58,20 +58,6 @@ namespace {{variant}} {
 #include "{{export_header}}"
 {%- endif %}
 
-{#--- WTF enum hashing #}
-{%- from "enum_macros.tmpl" import enum_hash_blink_forward%}
-{%- if for_blink %}
-{%-   if all_enums %}
-#include "third_party/blink/renderer/platform/wtf/hash_functions.h"
-{%-   endif %}
-
-{%-   for enum in all_enums %}
-{%-     if not enum|is_native_only_kind %}
-{{enum_hash_blink_forward(enum)}}
-{%-     endif %}
-{%-   endfor %}
-{%- endif %}
-
 {%- if variant %}
 {%-   for namespace in namespaces_as_array %}
 namespace {{namespace}} {
@@ -91,6 +77,20 @@ using {{enum|get_name_for_kind(flatten_nested_kind=True)}} = mojo::NativeEnum;
 {%-   endfor %}
 {%- endif %}
 
+{#--- WTF enum hashing #}
+{%- from "enum_macros.tmpl" import enum_hash_blink_forward%}
+{%- if for_blink %}
+{%-   if all_enums %}
+#include "third_party/blink/renderer/platform/wtf/hash_functions.h"
+{%-   endif %}
+
+{%-   for enum in all_enums %}
+{%-     if not enum|is_native_only_kind %}
+{{enum_hash_blink_forward(enum)}}
+{%-     endif %}
+{%-   endfor %}
+{%- endif %}
+
 {{namespace_begin()}}
 
 {#- These are non-variant header only. #}