Refactors DimWindow and moves to ash/common

ash/ash.gyp

Index: ash/ash.gyp
diff --git a/ash/ash.gyp b/ash/ash.gyp
index 2702576bf29c6e7a4617470f66fc1507c19666a5..5db35af7d2df916b0c11294d942c7ad84e8999be 100644
--- a/ash/ash.gyp
+++ b/ash/ash.gyp
@@ -483,6 +483,8 @@
       'common/wm/window_cycle_event_filter.h',
       'common/wm/window_cycle_list.cc',
       'common/wm/window_cycle_list.h',
+      'common/wm/window_dimmer.cc',
+      'common/wm/window_dimmer.h',
       'common/wm/window_parenting_utils.cc',
       'common/wm/window_parenting_utils.h',
       'common/wm/window_positioner.cc',
@@ -538,6 +540,7 @@
       'common/wm_shell.cc',
       'common/wm_shell.h',
       'common/wm_transient_window_observer.h',
+      'common/wm_window.cc',
       'common/wm_window.h',
       'common/wm_window_observer.h',
       'common/wm_window_property.h',
@@ -722,8 +725,6 @@
       'wm/boot_splash_screen_chromeos.h',
       'wm/cursor_manager_chromeos.cc',
       'wm/cursor_manager_chromeos.h',
-      'wm/dim_window.cc',
-      'wm/dim_window.h',
       'wm/drag_window_controller.cc',
       'wm/drag_window_controller.h',
       'wm/drag_window_resizer.cc',