[ MimeHandlerView ] Reset beforeunload binding

When a new BeforeUnloadControl binding is being bound we should clear the old
binding. It is possible to try to bind an existing binding when the page is
refreshed.

TBR=wjmaclean@chromium.org

Bug: 659750
Change-Id: I1b41deb91e32cf394e9fa9c53de75c69141a8da5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1607503Reviewed-by: James MacLean <wjmaclean@chromium.org>
Commit-Queue: Ehsan Karamad <ekaramad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#658893}

extensions/renderer/guest_view/mime_handler_view/mime_handler_view_container_manager.cc

@@ -128,6 +128,10 @@ void MimeHandlerViewContainerManager::CreateBeforeUnloadControl(
   if (!post_message_support_)
     post_message_support_ = std::make_unique<PostMessageSupport>(this);
   mime_handler::BeforeUnloadControlPtr before_unload_control;
+  if (before_unload_control_binding_.is_bound()) {
+    // Might happen when reloading the same page.
+    before_unload_control_binding_.Close();
+  }
   before_unload_control_binding_.Bind(
       mojo::MakeRequest(&before_unload_control));
   std::move(callback).Run(std::move(before_unload_control));