Mojo C++ bindings: support enum validation.

mojo/public/tools/bindings/generators/cpp_templates/interface_declaration.tmpl

 {%- import "interface_macros.tmpl" as interface_macros %}
 class {{interface.name}}Proxy;
 class {{interface.name}}Stub;
 
 class {{interface.name}}RequestValidator;
 {%- if interface|has_callbacks %}
 class {{interface.name}}ResponseValidator;
 {%- endif %}
 
 class {{interface.name}} {
@@ skipping 10 matching lines @@
   using RequestValidator_ = {{interface.name}}RequestValidator;
 {%- if interface|has_callbacks %}
   using ResponseValidator_ = {{interface.name}}ResponseValidator;
 {%- else %}
   using ResponseValidator_ = mojo::PassThroughFilter;
 {%- endif %}
 
 {#--- Enums #}
 {% from "enum_macros.tmpl" import enum_decl -%}
 {%- for enum in interface.enums %}
-  {{enum_decl(enum, is_static=true)|indent(2)}}
+  {{enum_decl(enum)|indent(2)}}
 {%- endfor %}
 
 {#--- Constants #}
 {%- for constant in interface.constants %}
 {%-   if constant.kind|is_integral_kind %}
   static const {{constant.kind|cpp_pod_type}} {{constant.name}} = {{constant|constant_value}};
 {%-   else %}
   static const {{constant.kind|cpp_pod_type}} {{constant.name}};
 {%-   endif %}
 {%- endfor %}
 
 {#--- Methods #}
   virtual ~{{interface.name}}() {}
 
 {%- for method in interface.methods %}
 {%    if method.response_parameters != None %}
   using {{method.name}}Callback = {{interface_macros.declare_callback(method)}};
 {%-   endif %}
   virtual void {{method.name}}({{interface_macros.declare_request_params("", method)}}) = 0;
 {%- endfor %}
 };