Mojo C++ bindings: support mapping mojo string to WTF::String.

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

 {%- macro declare_params(prefix, parameters) %}
 {%-   for param in parameters -%}
 {{param.kind|cpp_const_wrapper_type}} {{prefix}}{{param.name}}
 {%- if not loop.last %}, {% endif %}
 {%-   endfor %}
 {%- endmacro %}
 
-{%- macro declare_callback(method) -%}
+{%- macro declare_callback(method, use_wtf_types) -%}
 mojo::Callback<void(
 {%-   for param in method.response_parameters -%}
 {#- TODO(yzshen): Previously |cpp_result_type| is used here incorrectly.
     The outcome is we will use mojo::String instead of const mojo::String&.
     Preserve the behavior temporarily. #}
-{%- if not param.kind|is_string_kind -%}
+{%- if (not param.kind|is_string_kind) or use_wtf_types -%}
 {{param.kind|cpp_const_wrapper_type}}
 {%- else -%}
 mojo::String
 {%- endif %}
 {%- if not loop.last %}, {% endif %}
 {%-   endfor -%}
 )>
 {%- endmacro -%}
 
 {%- macro declare_request_params(prefix, method) -%}
 {{declare_params(prefix, method.parameters)}}
 {%-   if method.response_parameters != None -%}
 {%- if method.parameters %}, {% endif -%}
 const {{method.name}}Callback& callback
 {%-   endif -%}
 {%- endmacro -%}
 
 {%- macro declare_sync_method_params(prefix, method) -%}
 {{declare_params(prefix, method.parameters)}}
 {%-   if method.response_parameters %}
 {%-     if method.parameters %}, {% endif %}
 {%-     for param in method.response_parameters -%}
 {{param.kind|cpp_wrapper_type}}* {{prefix}}{{param.name}}
 {%-       if not loop.last %}, {% endif %}
 {%-     endfor %}
 {%-   endif -%}
 {%- endmacro -%}