Adding a gray semi transparent backdrop behind the topmost window within the default container

ash/ash.gyp

 # Copyright (c) 2012 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.
 
 {
   'variables': {
     'chromium_code': 1,
     'grit_out_dir': '<(SHARED_INTERMEDIATE_DIR)/chrome',
   },
   'includes': [
@@ skipping 618 matching lines @@
         'wm/workspace/multi_window_resize_controller.h',
         'wm/workspace/phantom_window_controller.cc',
         'wm/workspace/phantom_window_controller.h',
         'wm/workspace/snap_sizer.cc',
         'wm/workspace/snap_sizer.h',
         'wm/workspace/snap_types.h',
         'wm/workspace/workspace_event_handler.cc',
         'wm/workspace/workspace_event_handler.h',
         'wm/workspace/workspace_layout_manager.cc',
         'wm/workspace/workspace_layout_manager.h',
+        'wm/workspace/workspace_layout_manager_backdrop.cc',
+        'wm/workspace/workspace_layout_manager_backdrop.h',
         'wm/workspace/workspace_types.h',
         'wm/workspace/workspace_window_resizer.cc',
         'wm/workspace/workspace_window_resizer.h',
       ],
       'conditions': [
         ['OS=="win"', {
           'sources/': [
             ['exclude', 'host/window_tree_host_factory.cc'],
             ['exclude', 'sticky_keys/sticky_keys_controller.cc'],
             ['exclude', 'sticky_keys/sticky_keys_controller.h'],
@@ skipping 463 matching lines @@
             },
           },
           'dependencies': [
             '../sandbox/sandbox.gyp:sandbox',
           ],
         }],
       ],
     },
   ],
 }