Allow passing listening socket path on command line

When enabling externally-running applications, allow the
user to specify a path for the listening socket. This makes
development easier while still allowing for a default
to be used in production.

BUG=None
TEST=run mojo_shell with --enable-external-applications=path/to/socket, without the flag, and with the flag but passing no value.

Review URL: https://codereview.chromium.org/599733007

Cr-Commit-Position: refs/heads/master@{#297247}

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_)));
   }