Mojo cpp bindings: remove redundant validation in Decode*().

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

 {%- import "struct_macros.tmpl" as struct_macros %}
 {%- set class_name = struct.name ~ "_Data" %}
 
 // static
 {{class_name}}* {{class_name}}::New(mojo::internal::Buffer* buf) {
   return new (buf->Allocate(sizeof({{class_name}}))) {{class_name}}();
 }
 
 // static
 bool {{class_name}}::Validate(const void* data,
                               mojo::internal::BoundsChecker* bounds_checker) {
   {{ struct_macros.validate(struct, class_name)|indent(2) }}
 }
 
 {{class_name}}::{{class_name}}() {
   header_.num_bytes = sizeof(*this);
   header_.num_fields = {{struct.packed.packed_fields|length}};
 }
 
 void {{class_name}}::EncodePointersAndHandles(
     std::vector<mojo::Handle>* handles) {
   {{ struct_macros.encodes(struct)|indent(2) }}
 }
 
-bool {{class_name}}::DecodePointersAndHandles(mojo::Message* message) {
+void {{class_name}}::DecodePointersAndHandles(
+    std::vector<mojo::Handle>* handles) {
   {{ struct_macros.decodes(struct)|indent(2) }}
-  return true;
 }