Creates BackgroundShell and uses it in test

mojo/shell/background/background_shell_main.h

Index: mojo/shell/background/background_shell_main.h
diff --git a/mojo/shell/background/background_shell_main.h b/mojo/shell/background/background_shell_main.h
new file mode 100644
index 0000000000000000000000000000000000000000..6b217d447806b514d089eeb0a48b743777449283
--- /dev/null
+++ b/mojo/shell/background/background_shell_main.h
@@ -0,0 +1,12 @@
+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef MOJO_SHELL_BACKGROUND_BACKGROUND_SHELL_MAIN_H_
+#define MOJO_SHELL_BACKGROUND_BACKGROUND_SHELL_MAIN_H_
+
+// The "main" gn target supplies a file with a main() that calls to the child
+// process as necessary. For the main process this function is called.
+int MasterProcessMain(int argc, char** argv);
+
+#endif  // MOJO_SHELL_BACKGROUND_BACKGROUND_SHELL_MAIN_H_