aura: Draw drop shadows under browsers and menus.

ui/aura_shell/aura_shell.gyp

Index: ui/aura_shell/aura_shell.gyp
diff --git a/ui/aura_shell/aura_shell.gyp b/ui/aura_shell/aura_shell.gyp
index 5639febabf62c7127ff710f42a552167232e887a..345b9992b1d4d14f183ccc7da35439e563063ba6 100644
--- a/ui/aura_shell/aura_shell.gyp
+++ b/ui/aura_shell/aura_shell.gyp
@@ -49,6 +49,8 @@
         'drag_drop_controller.h',
         'drag_image_view.cc',
         'drag_image_view.h',
+        'image_grid.cc',
+        'image_grid.h',
         'launcher/app_launcher_button.cc',
         'launcher/app_launcher_button.h',
         'launcher/launcher.cc',
@@ -72,6 +74,10 @@
         'modality_event_filter_delegate.h',
         'property_util.cc',
         'property_util.h',
+        'shadow.cc',
+        'shadow.h',
+        'shadow_controller.cc',
+        'shadow_controller.h',
         'shelf_layout_controller.cc',
         'shelf_layout_controller.h',
         'shell.cc',
@@ -129,11 +135,13 @@
         'default_container_layout_manager_unittest.cc',
         'desktop_event_filter_unittest.cc',
         'drag_drop_controller_unittest.cc',
+        'image_grid_unittest.cc',
         'launcher/launcher_model_unittest.cc',
         'launcher/view_model_unittest.cc',
         'launcher/view_model_utils_unittest.cc',
         'modal_container_layout_manager_unittest.cc',
         'run_all_unittests.cc',
+        'shadow_controller_unittest.cc',
         'shell_unittest.cc',
         'stacking_controller_unittest.cc',
         'test_suite.cc',