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

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

 {%- import "struct_macros.tmpl" as struct_macros %}
 {%- set class_name = struct.name ~ "_Data" -%}
 
 class {{class_name}} {
  public:
   static {{class_name}}* New(mojo::internal::Buffer* buf);
 
   static bool Validate(const void* data,
                        mojo::internal::BoundsChecker* bounds_checker);
 
   mojo::internal::StructHeader header_;
 {{struct_macros.fields(struct)}}
 
   void EncodePointersAndHandles(std::vector<mojo::Handle>* handles);
-  bool DecodePointersAndHandles(mojo::Message* message);
+  void DecodePointersAndHandles(std::vector<mojo::Handle>* handles);
 
  private:
   {{class_name}}();
   ~{{class_name}}();  // NOT IMPLEMENTED
 };
 MOJO_COMPILE_ASSERT(sizeof({{class_name}}) == {{struct.packed|struct_size}},
                     bad_sizeof_{{class_name}});