Mojo C++ bindings: switch the existing usage of StructTraits to use the new data view interface.

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.
     |context| is the name of the serialization context.
     |input_may_be_temp| indicates whether any input may be temporary obejcts.
     We need to assign temporary objects to local variables before passing it to
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     placeholder, for example, "result->%s", "p_%s". The placeholder will be
     substituted with struct field names to refer to the output fields.
     |context| is the name of the serialization context.
     |success| is the name of a bool variable to track success of the operation.
     This macro is expanded to do deserialization for both:
     - user-defined structs: the output is an instance of the corresponding
       struct wrapper class.
     - method parameters/response parameters: the output is a list of
       arguments. #}
 {%- macro deserialize(struct, input, output_field_pattern, success) -%}
-  DCHECK(!{{input}}.is_null());
-
 {%-   for pf in struct.packed.packed_fields_in_ordinal_order %}
 {%-     set output_field = output_field_pattern|format(pf.field.name) %}
 {%-     set name = pf.field.name %}
 {%-     set kind = pf.field.kind %}
 {%-     if kind|is_object_kind %}
   if (!{{input}}.Read{{name|under_to_camel}}(&{{output_field}}))
     {{success}} = false;
 {%-     elif kind|is_interface_kind or kind|is_any_handle_kind or
              kind|is_associated_kind %}
   {{output_field}} = {{input}}.Take{{name|under_to_camel}}();
 {%-     else %}
   {{output_field}} = {{input}}.{{name}}();
 {%-     endif %}
 {%-   endfor %}
 {%- endmacro %}