[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 c04b9a40756d478f47baed1fc6b64fbf976d7ce4..07c38e32cbfbbe05a037f9b173afe434f9f67353 100644
--- a/apps/app_shim/chrome_main_app_mode_mac.mm
+++ b/apps/app_shim/chrome_main_app_mode_mac.mm
@@ -621,7 +621,11 @@ 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);