Use the delegate mechanism on ash to support both aura and mus.

This eliminates aura::Window usage in SystemTrayBubble and WebNotificationTray,
which is step toward converting TrayBubbleView to wm common types.

I didn't eliminate BubbleDialogDelegateView::parent_window_ because that
mechanism is used in ~30 places inside //chrome.

BUG=619636
TEST=ash_unittests, views_unittests, manually check that system tray bubbles
still open/close propertly

Review-Url: https://codereview.chromium.org/2092473002
Cr-Commit-Position: refs/heads/master@{#401469}