Generate a templated Clone method for all mojo structs.

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

 {% from "enum_macros.tmpl" import enum_decl -%}
 
 class {{struct.name}} {
  public:
   using Data_ = internal::{{struct.name}}_Data;
 
 {#--- Enums #}
 {%- for enum in struct.enums -%}
   {{enum_decl(enum)|indent(2)}}
 {%- endfor %}
@@ skipping 15 matching lines @@
   }
 
   template <typename U>
   U To() const {
     return mojo::TypeConverter<U, {{struct.name}}>::Convert(*this);
   }
 
   {{struct.name}}();
   ~{{struct.name}}();
 
-{%  if struct|is_cloneable_kind %}
-  {{struct.name}}Ptr Clone() const;
-{%- endif %}
+  // Clone() is a template so it is only instantiated if it is used. Thus, the
+  // bindings generator does not need to know whether typemapped native types
+  // support operator=.
+  template <typename StructPtrType = {{struct.name}}Ptr>
+  {{struct.name}}Ptr Clone() const {
+    // Use StructPtrType to prevent the compiler from trying to compile this
+    // without being asked.
+    StructPtrType rv(New());
+{%-   for field in struct.fields %}
+{%-     if field.kind|is_object_kind and not field.kind|is_string_kind and
+            not field.kind|is_typemapped_kind %}
+    rv->{{field.name}} = {{field.name}}.Clone();
+{%-     else %}
+    rv->{{field.name}} = {{field.name}};
+{%-     endif %}
+{%-   endfor %}
+    return rv;
+  }
+
   // Equals() is a template so it is only instantiated if it is used. Thus, the
   // bindings generator does not need to know whether typemapped native types
   // support operator==.
   template <typename T,
             typename std::enable_if<std::is_same<
                 T, {{struct.name}}>::value>::type* = nullptr>
   bool Equals(const T& other) const {
   {%-  for field in struct.fields %}
     if (!mojo::internal::ValueTraits<{{field.kind|cpp_wrapper_type}}>::Equals(this->{{field.name}}, other.{{field.name}}))
       return false;
   {%-  endfor %}
     return true;
   }
 
 {#--- Struct members #}
 {%  for field in struct.fields %}
 {%-   set type = field.kind|cpp_wrapper_type %}
 {%-   set name = field.name %}
   {{type}} {{name}};
 {%- endfor %}
 };