Shell: Remove the test child stuff.

shell/app_child_process.mojom

Index: shell/app_child_process.mojom
diff --git a/shell/app_child_process.mojom b/shell/app_child_process.mojom
index 3da353d9bf3f4fe9c364344330f690f4f7b1996e..4b7480d23e6e34b60e7e96d7274aa5e615115fc9 100644
--- a/shell/app_child_process.mojom
+++ b/shell/app_child_process.mojom
@@ -7,7 +7,11 @@ module mojo.shell;
 import "mojo/public/interfaces/application/application.mojom";
 
 interface AppChildController {
+  // Starts the app at the given path (deleting it if |clean_app_path| is true).
   StartApp(string app_path,
            bool clean_app_path,
            mojo.Application& application_request) => (int32 result);
+
+  // Exits the child process now (with no cleanup), with the given exit code.
+  ExitNow(int32 exit_code);
 };