[fuchsia][engine] Fix message port binding in FrameImplTest

FrameImplTest.PostMessageBadOriginDropped tests posting a message from
an invalid origin. This should result in no message being received, but
the test was accidentally attempting to receive a message from an
unbound fuchsia::web::MessagePortPtr. This updates the test to use a
bound handle and validate that no message is received on this channel.

Test: web_engine_browsertests
Bug: fuchsia:63593
Change-Id: I84f532fff62ea4dc9c6741a7872f181e530935d3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2530219
Auto-Submit: James Robinson <jamesr@chromium.org>
Commit-Queue: Wez <wez@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826206}

fuchsia/engine/browser/frame_impl_browsertest.cc

@@ -1491,18 +1491,15 @@ IN_PROC_BROWSER_TEST_F(FrameImplTest, PostMessageBadOriginDropped) {
   navigation_listener_.RunUntilUrlAndTitleEquals(post_message_url,
                                                  "messageport");
 
-  fuchsia::web::MessagePortPtr bad_origin_incoming_message_port;
-  fuchsia::web::WebMessage msg;
-
   // PostMessage() to invalid origins should be ignored. We pass in a
   // MessagePort but nothing should happen to it.
-  fuchsia::web::MessagePortPtr unused_message_port;
+  fuchsia::web::MessagePortPtr bad_origin_incoming_message_port;
   cr_fuchsia::ResultReceiver<fuchsia::web::Frame_PostMessage_Result>
       unused_post_result;
   frame->PostMessage(
       "https://example.com",
       cr_fuchsia::CreateWebMessageWithMessagePortRequest(
-          unused_message_port.NewRequest(),
+          bad_origin_incoming_message_port.NewRequest(),
           cr_fuchsia::MemBufferFromString("bad origin, bad!", "test")),
       cr_fuchsia::CallbackToFitFunction(
           unused_post_result.GetReceiveCallback()));