C++ bindings: A struct's Deserialize() now does validation before deserializing.

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

 {#  TODO(yzshen): Make these templates more readable. #}
 
 {#  Computes the serialized size for the specified struct.
     |struct| is the struct definition.
     |input_field_pattern| should be a pattern that contains one string
     placeholder, for example, "input->%s", "p_%s". The placeholder will be
     substituted with struct field names to refer to the input fields.
     This macro is expanded to compute seriailized size for both:
     - user-defined structs: the input is an instance of the corresponding struct
       wrapper class.
@@ skipping 262 matching lines @@
    all generated structs:
    - user-defined mojo structs
    - interface method params/response params #}
 {%- macro structptr_forward_decl(struct) -%}
 class {{struct.name}};
 {%    if struct|should_inline %}
 using {{struct.name}}Ptr = mojo::InlinedStructPtr<{{struct.name}}>;
 {%    else %}
 using {{struct.name}}Ptr = mojo::StructPtr<{{struct.name}}>;
 {%    endif %}
-{% endmacro %}
+{% endmacro %}