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

mojo/public/tools/bindings/generators/cpp_templates/module.cc.tmpl

Index: mojo/public/tools/bindings/generators/cpp_templates/module.cc.tmpl
diff --git a/mojo/public/tools/bindings/generators/cpp_templates/module.cc.tmpl b/mojo/public/tools/bindings/generators/cpp_templates/module.cc.tmpl
index 447b6a4a9fe7ecaf389638e7739937a5e195a67f..457fba4bbb3ad8f75a07ba6b30630781a294285a 100644
--- a/mojo/public/tools/bindings/generators/cpp_templates/module.cc.tmpl
+++ b/mojo/public/tools/bindings/generators/cpp_templates/module.cc.tmpl
@@ -36,6 +36,10 @@
 #include "mojo/public/cpp/environment/lib/scoped_task_tracking.h"
 #include "mojo/public/interfaces/bindings/interface_control_messages.mojom.h"
 
+{%- if use_wtf_types %}
+#include "mojo/public/cpp/bindings/lib/wtf_serialization.h"
+{%- endif %}
+
 {%- for namespace in namespaces_as_array %}
 namespace {{namespace}} {
 {%- endfor %}