Allow passing listening socket path on command line

mojo/shell/context.cc

Index: mojo/shell/context.cc
diff --git a/mojo/shell/context.cc b/mojo/shell/context.cc
index 253f4b7d24d85bf6b2fc2498b2151b2ef238a697..6aa84f809d2d006bf9fa62f1d567ffd260558640 100644
--- a/mojo/shell/context.cc
+++ b/mojo/shell/context.cc
@@ -185,8 +185,14 @@ void Context::Init() {
   if (command_line->HasSwitch(switches::kEnableExternalApplications)) {
     listener_ = ExternalApplicationListener::Create(
         task_runners_->shell_runner(), task_runners_->io_runner());
+
+    base::FilePath socket_path =
+        command_line->GetSwitchValuePath(switches::kEnableExternalApplications);
+    if (socket_path.empty())
+      socket_path = ExternalApplicationListener::ConstructDefaultSocketPath();
+
     listener_->ListenInBackground(
-        ExternalApplicationListener::ConstructDefaultSocketPath(),
+        socket_path,
         base::Bind(&ApplicationManager::RegisterExternalApplication,
                    base::Unretained(&application_manager_)));
   }