Mojo C++ bindings: switch ios/web mojom targets to use STL types.

ios/web/webui/web_ui_mojo_inttest.mm

Index: ios/web/webui/web_ui_mojo_inttest.mm
diff --git a/ios/web/webui/web_ui_mojo_inttest.mm b/ios/web/webui/web_ui_mojo_inttest.mm
index da7c3cf31a52880a23be3ad356778d18ace83c92..a223c33dc159869ec22196ad36fc3262cb14a2c3 100644
--- a/ios/web/webui/web_ui_mojo_inttest.mm
+++ b/ios/web/webui/web_ui_mojo_inttest.mm
@@ -49,16 +49,16 @@ class TestUIHandler
 
   // TestUIHandlerMojo overrides.
   void SetClientPage(TestPagePtr page) override { page_ = std::move(page); }
-  void HandleJsMessage(const mojo::String& message) override {
-    if (message.get() == "syn") {
+  void HandleJsMessage(const std::string& message) override {
+    if (message == "syn") {
       // Received "syn" message from WebUI page, send "ack" as reply.
       DCHECK(!syn_received_);
       DCHECK(!fin_received_);
       syn_received_ = true;
       NativeMessageResultMojoPtr result(NativeMessageResultMojo::New());
-      result->message = mojo::String::From("ack");
+      result->message = "ack";
       page_->HandleNativeMessage(std::move(result));
-    } else if (message.get() == "fin") {
+    } else if (message == "fin") {
       // Received "fin" from the WebUI page in response to "ack".
       DCHECK(syn_received_);
       DCHECK(!fin_received_);