SequenceBound: migrate from Post() to AsyncCall() in remoting classes.

Bug: 1140588
Change-Id: I0fd8db6f30a42205dafe889043012e3de4a4a2ef
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2487827Reviewed-by: Lambros Lambrou <lambroslambrou@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#819481}

remoting/host/file_transfer/file_chooser_linux.cc

@@ -159,8 +159,7 @@ FileChooserLinux::FileChooserLinux(
 }
 
 void FileChooserLinux::Show() {
-  gtk_file_chooser_on_ui_thread_.Post(FROM_HERE,
-                                      &GtkFileChooserOnUiThread::Show);
+  gtk_file_chooser_on_ui_thread_.AsyncCall(&GtkFileChooserOnUiThread::Show);
 }
 
 void FileChooserLinux::RunCallback(FileChooser::Result result) {