mojo: Mark DataView readers as WARN_UNUSED_RESULT.

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

 class {{union.name}}DataView {
  public:
   using Tag = internal::{{union.name}}_Data::{{union.name}}_Tag;
 
   {{union.name}}DataView() {}
 
   {{union.name}}DataView(
       internal::{{union.name}}_Data* data,
       mojo::internal::SerializationContext* context)
 {%- if union|requires_context_for_data_view %}
@@ skipping 13 matching lines @@
 {%- for field in union.fields %}
 {%-   set kind = field.kind %}
 {%-   set name = field.name %}
   bool is_{{name}}() const { return data_->tag == Tag::{{name|upper}}; }
 
 {%-   if kind|is_object_kind %}
   inline void Get{{name|under_to_camel}}DataView(
       {{kind|cpp_data_view_type}}* output);
 
   template <typename UserType>
-  bool Read{{name|under_to_camel}}(UserType* output) {
+  WARN_UNUSED_RESULT bool Read{{name|under_to_camel}}(UserType* output) {
     DCHECK(is_{{name}}());
     return mojo::internal::Deserialize<{{kind|unmapped_type_for_serializer}}>(
         data_->data.f_{{name}}.Get(), output, context_);
   }
 
 {%-   elif kind|is_enum_kind %}
   template <typename UserType>
-  bool Read{{name|under_to_camel}}(UserType* output) const {
+  WARN_UNUSED_RESULT bool Read{{name|under_to_camel}}(UserType* output) const {
     DCHECK(is_{{name}}());
     return mojo::internal::Deserialize<{{kind|unmapped_type_for_serializer}}>(
         data_->data.f_{{name}}, output);
   }
 
   {{kind|cpp_data_view_type}} {{name}}() const {
     DCHECK(is_{{name}}());
     return static_cast<{{kind|cpp_data_view_type}}>(
         data_->data.f_{{name}});
   }
@@ skipping 30 matching lines @@
 {%-   endif %}
 {%- endfor %}
 
  private:
   internal::{{union.name}}_Data* data_ = nullptr;
 {%- if union|requires_context_for_data_view %}
   mojo::internal::SerializationContext* context_ = nullptr;
 {%- endif %}
 };