Move application_manager's ShellImpl to own file

mojo/application_manager/BUILD.gn

 # Copyright 2014 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.
 
 import("//mojo/public/tools/bindings/mojom.gni")
 
 # GYP version: mojo.gyp:mojo_application_manager
 component("application_manager") {
   output_name = "mojo_application_manager"
   sources = [
     "application_loader.cc",
     "application_loader.h",
     "application_manager.cc",
     "application_manager.h",
     "application_manager_export.h",
+    "shell_impl.cc",
+    "shell_impl.h",
   ]
 
   defines = [ "MOJO_APPLICATION_MANAGER_IMPLEMENTATION" ]
 
   public_deps = [
     "//base",
     "//mojo/common",
     "//mojo/public/interfaces/application:application",
     "//mojo/services/network/public/interfaces",
     "//url",
@@ skipping 27 matching lines @@
     "//testing/gtest",
     "//url",
   ]
 }
 
 mojom("test_bindings") {
   sources = [
     "test.mojom",
   ]
 }