Mojo C++ bindings: switch services/navigation mojom target to use STL types.

mash/webtest/webtest.cc

Index: mash/webtest/webtest.cc
diff --git a/mash/webtest/webtest.cc b/mash/webtest/webtest.cc
index ecf223282dd94ed644f1ef049165581623507cf6..83958647191bf31189b73901c212e6e794643e81 100644
--- a/mash/webtest/webtest.cc
+++ b/mash/webtest/webtest.cc
@@ -103,10 +103,10 @@ class UI : public views::WidgetDelegateView,
   void OpenURL(navigation::mojom::OpenURLParamsPtr params) override {}
   void LoadingStateChanged(bool is_loading) override {}
   void NavigationStateChanged(const GURL& url,
-                              const mojo::String& title,
+                              const std::string& title,
                               bool can_go_back,
                               bool can_go_forward) override {
-    current_title_ = base::UTF8ToUTF16(title.get());
+    current_title_ = base::UTF8ToUTF16(title);
     GetWidget()->UpdateWindowTitle();
   }
   void LoadProgressChanged(double progress) override {}