[mus+ash] Removes WmWindow from ash/wm/overview and ash/wm/workspace

BUG=722553
TEST=None (refactoring)

Review-Url: https://codereview.chromium.org/2908793002
Cr-Commit-Position: refs/heads/master@{#475096}

• ash/aura/shell_port_classic.cc[diff]
• ash/aura/shell_port_classic.h[diff]
• ash/mus/bridge/shell_port_mash.cc[diff]
• ash/mus/bridge/shell_port_mash.h[diff]
• ash/shelf/shelf_layout_manager.cc[diff]
• ash/shelf/shelf_tooltip_manager.cc[diff]
• ash/shelf/shelf_window_watcher.cc[diff]
• ash/shelf/shelf_window_watcher_item_delegate.cc[diff]
• ash/shelf/shelf_window_watcher_item_delegate.h[diff]
• ash/shell.cc[diff]
• ash/shell_port.h[diff]
• ash/wm/overview/cleanup_animation_observer_unittest.cc[diff]
• ash/wm/overview/scoped_transform_overview_window.cc[diff]
• ash/wm/overview/window_grid.cc[diff]
• ash/wm/overview/window_selector.cc[diff]
• ash/wm/overview/window_selector_controller.cc[diff]
• ash/wm/overview/window_selector_item.cc[diff]
• ash/wm/overview/window_selector_unittest.cc[diff]
• ash/wm/panels/panel_layout_manager_unittest.cc[diff]
• ash/wm/tablet_mode/tablet_mode_event_handler.cc[diff]
• ash/wm/tablet_mode/tablet_mode_window_manager_unittest.cc[diff]
• ash/wm/tablet_mode/tablet_mode_window_state.cc[diff]
• ash/wm/window_util.cc[diff]
• ash/wm/window_util.h[diff]
• ash/wm/workspace/backdrop_controller.cc[diff]
• ash/wm/workspace/multi_window_resize_controller_unittest.cc[diff]
• ash/wm/workspace/workspace_event_handler_aura.cc[diff]
• ash/wm/workspace/workspace_event_handler_aura.h[diff]
• ash/wm/workspace/workspace_layout_manager.cc[diff]
• ash/wm/workspace/workspace_layout_manager_keyboard_unittest.cc[diff]
• ash/wm/workspace/workspace_layout_manager_unittest.cc[diff]
• ash/wm/workspace/workspace_window_resizer_unittest.cc[diff]
• ash/wm/workspace_controller.cc[diff]
• ash/wm_window.cc[diff]
• ash/wm_window.h[diff]