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 8c9c0fb0769685c3ab4de2eb896937037d529ac2..4d1dc90eece181bf0c0425098879d6bdfe1cfeac 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/bindings/lib/validation_util.h"
 #include "mojo/public/interfaces/bindings/interface_control_messages.mojom.h"
 
+{%- if for_blink %}
+#include "mojo/public/cpp/bindings/lib/wtf_serialization.h"
+{%- endif %}
+
 {%- for namespace in namespaces_as_array %}
 namespace {{namespace}} {
 {%- endfor %}