Creates BackgroundShell and uses it in test

mojo/shell/background/BUILD.gn

+# 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.
+
+source_set("lib") {
+  sources = [
+    "background_shell.cc",
+    "background_shell.h",
+  ]
+  deps = [
+    "//base",
+    "//mojo/message_pump",
+    "//mojo/shell",
+    "//mojo/shell/public/cpp:sources",
+    "//mojo/shell/standalone:lib",
+  ]
+}
+
+source_set("main") {
+  sources = [
+    "background_shell_main.cc",
+    "background_shell_main.h",
+  ]
+  deps = [
+    "//base",
+    "//mojo/message_pump",
+    "//mojo/shell",
+    "//mojo/shell/public/cpp:sources",
+    "//mojo/shell/runner/common",
+    "//mojo/shell/runner/host:lib",
+    "//mojo/shell/standalone:lib",
+  ]
+}