Add operator<< for mojo::String for logging use

mojo/public/cpp/bindings/string.h

Index: mojo/public/cpp/bindings/string.h
diff --git a/mojo/public/cpp/bindings/string.h b/mojo/public/cpp/bindings/string.h
index 03080c8376483eec2ae91c5352aede73bb7afcc2..872dbca87f237869a50b3ebcbf6990ee332ee59a 100644
--- a/mojo/public/cpp/bindings/string.h
+++ b/mojo/public/cpp/bindings/string.h
@@ -106,6 +106,10 @@ inline bool operator!=(const String& a, const String& b) { return !(a == b); }
 inline bool operator!=(const char* a, const String& b) { return !(a == b); }
 inline bool operator!=(const String& a, const char* b) { return !(a == b); }
 
+inline std::ostream& operator<<(std::ostream& out, const String& s) {
+  return out << s.get();
+}
+
 // TODO(darin): Add similar variants of operator<,<=,>,>=
 
 template <>