[Mac] Add interactive App Shim test.

apps/app_shim/chrome_main_app_mode_mac.mm

Index: apps/app_shim/chrome_main_app_mode_mac.mm
diff --git a/apps/app_shim/chrome_main_app_mode_mac.mm b/apps/app_shim/chrome_main_app_mode_mac.mm
index ee81b77913540ec0d22db275e58de5edbeba324d..18b8f9e1cfdbca32b371e69ba850be1f2bc33587 100644
--- a/apps/app_shim/chrome_main_app_mode_mac.mm
+++ b/apps/app_shim/chrome_main_app_mode_mac.mm
@@ -611,7 +611,11 @@ int ChromeAppModeStart(const app_mode::ChromeAppModeInfo* info) {
   main_message_loop.set_thread_name("MainThread");
   base::PlatformThread::SetName("CrAppShimMain");
 
-  if (pid == -1) {
+  // In tests, launching Chrome does nothing, and we won't get a ping response,
+  // so just assume the socket exists.
+  if (pid == -1 &&
+      !CommandLine::ForCurrentProcess()->HasSwitch(
+          app_mode::kLaunchedForTest)) {
     // Launch Chrome if it isn't already running.
     ProcessSerialNumber psn;
     CommandLine command_line(CommandLine::NO_PROGRAM);