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

mojo/public/cpp/bindings/lib/bindings_internal.h

Index: mojo/public/cpp/bindings/lib/bindings_internal.h
diff --git a/mojo/public/cpp/bindings/lib/bindings_internal.h b/mojo/public/cpp/bindings/lib/bindings_internal.h
index a467acdd0d6f97f5cbef764db9ff419e6a23064c..8d320823139e21835f8cae7cd976a00ed6395da8 100644
--- a/mojo/public/cpp/bindings/lib/bindings_internal.h
+++ b/mojo/public/cpp/bindings/lib/bindings_internal.h
@@ -11,6 +11,9 @@
 #include "mojo/public/cpp/bindings/lib/template_util.h"
 #include "mojo/public/cpp/system/core.h"
 
+namespace WTF {
+class String;
+}
 namespace mojo {
 class String;
 
@@ -162,6 +165,11 @@ struct WrapperTraits<String, false> {
   typedef String_Data* DataType;
 };
 
+template <>
+struct WrapperTraits<WTF::String, false> {
+  typedef String_Data* DataType;
+};
+
 }  // namespace internal
 }  // namespace mojo