Revert of mash: Convert TrayBackgroundView to wm common types

Revert of mash: Convert TrayBackgroundView to wm common types (patchset #4 id:60001 of https://codereview.chromium.org/2091933002/ )

Reason for revert:
Broke compile. Looks like this was conflicting with https://chromium.googlesource.com/chromium/src/+/133c792af9dfe8c82561bee7015efcc0b5dba22d landed just before. Broken build: https://build.chromium.org/p/chromium.chromiumos/builders/Linux%20ChromiumOS%20Builder%20%28dbg%29/builds/76385

Original issue's description:
> mash: Convert TrayBackgroundView to wm common types
> 
> Also convert StatusAreaWidget usage to WmShelf. This is closer to the long-
> term pattern we want, where StatusAreaWidget is accessed via wm types.
> 
> * Eliminate the TrayBackgroundView::status_area_widget() accessors
> * Cache a copy of StatusAreaWidget* in subclasses where needed
> 
> Move the following interdependent files:
> move_source_file.py ash/system/tray/tray_background_view.* ash/common/system/tray/
> move_source_file.py ash/system/tray/tray_bubble_wrapper.* ash/common/system/tray/
> move_source_file.py ash/system/tray/tray_event_filter.* ash/common/system/tray/
> move_source_file.py ash/system/tray/system_tray_bubble.* ash/common/system/tray/
> 
> BUG=619636
> TEST=ash_unittests
> TBR=stevenjb@chromium.org for 1 line rename in c/b/ui/chromeos/
> 
> Committed: https://crrev.com/f6f6340048f0ec9535c310d16dd515133fb04c50
> Cr-Commit-Position: refs/heads/master@{#401769}

TBR=msw@chromium.org,stevenjb@chromium.org,jamescook@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=619636

[dgozman, 2016-06-24 02:36:23]

The CQ bit was checked by dgozman@chromium.org

[dgozman, 2016-06-24 02:36:24]

Created Revert of mash: Convert TrayBackgroundView to wm common types

[dgozman, 2016-06-24 02:36:43]

The CQ bit was unchecked by dgozman@chromium.org