The previous version of this patch, https://codereview.chromium.org/2091933002
collided during commit with https://codereview.chromium.org/2087703002. This
is just a rebase.

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=msw@chromium.org

Review-Url: https://codereview.chromium.org/2099603002
Cr-Commit-Position: refs/heads/master@{#401820}